Monster Cookie Bars

These Monster Cookie Bars are chewy, peanut-butter slabs with an oat-forward bite and pockets of chocolate from M&Ms and chips. The surface browns around the edges while the center stays tender, and optional candy eyeballs make them a playful Halloween treat.

They come together with a quick mixing step, then chill for at least 2 hours or overnight before baking. The slab bakes in about 33 to 37 minutes, making these ideal for bake sales, lunchboxes, or a party-ready tray of squares.

Why You’ll Love These Bars

A few practical reasons these slab-style cookies are a go-to for parties and make-ahead baking:

Make-ahead: chilling the assembled slab gives clean, even slices and reliable texture.
Chewy, oat-forward texture from old-fashioned rolled oats balanced by creamy peanut butter.
Colorful, kid-friendly pockets of chocolate and crunch from M&Ms and chocolate chips.
Easy to dress up for Halloween—top with candy eyeballs for instant spooky appeal.

Follow the simple chilling and bake cues—350 degrees F and 33 to 37 minutes with the top puffed and edges just browning—for consistent results every time.

Ingredients You’ll Need

1/2 cup unsalted butter, 113g (1 stick), at room temperature: Unsalted butter provides the fat and tenderness that gives the bars a tender crumb; room temperature butter creams smoothly with the sugar.
1 cup light brown sugar, 210g, packed: Packed light brown sugar adds moisture and chewiness plus a hint of molasses that complements the peanut butter.
1/2 cup creamy peanut butter, 140g, at room temperature: Creamy peanut butter supplies the signature nutty flavor and contributes to the slab’s chew and richness; room temperature helps it blend evenly.
1 teaspoon honey: Honey adds subtle sticky sweetness and helps keep the bars soft as they bake.
1 1/2 teaspoons vanilla extract: Vanilla extract boosts overall flavor and rounds out the peanut-butter and brown-sugar notes.
1 large egg, at room temperature: The whole egg provides structure and moisture so the slab holds together when sliced.
1 large egg yolk, at room temperature: An extra egg yolk adds richness and chewiness to the bars’ interior texture.
1 cup all-purpose flour, 125g: All-purpose flour is the primary structure builder, keeping the bars from spreading too thin while letting them stay tender.
1/4 teaspoon fine sea salt: Fine sea salt balances sweetness and brightens the peanut-butter and chocolate flavors.
1/4 teaspoon baking soda: Baking soda gives a modest lift so the top puffs slightly without making the slab cake-like.
1 1/2 cups old-fashioned rolled oats, 150g: Old-fashioned rolled oats create the chewy, slightly toothsome texture that defines these monster bars.
1/2 cup M&Ms candies, 100g: M&Ms add colorful crunch and pockets of chocolate distributed through the slab.
1/2 cup chocolate chips, 90g: Chocolate chips provide soft, melty pockets of chocolate that complement the M&Ms.
candy eyeballs, for topping (optional): Candy eyeballs are an optional decoration that instantly turns the bars into Halloween-ready monsters.

How to Make Monster Cookie Bars

Lightly butter the bottom and sides of an 8-by-8-inch baking pan, then line the pan with parchment paper so it stays in place while you press in the dough.
In a mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the unsalted butter, packed light brown sugar, creamy peanut butter, honey, and vanilla until the mixture is fluffy and slightly lighter in color.
Scrape down the sides of the bowl, then add the egg and egg yolk and mix until fully incorporated.
Add the all-purpose flour, fine sea salt, and baking soda and mix on low speed until nearly incorporated. Add the old-fashioned rolled oats and mix briefly just to distribute them, then fold in the M&Ms and chocolate chips with a rubber spatula, scraping any dry bits from the bottom and sides of the bowl.
Transfer the dough to the prepared pan and use a spatula to press it evenly all the way to the edges. Cover the pan and chill for at least 2 hours or overnight; this firms the slab so you’ll get clean slices after baking.
Preheat the oven to 350 degrees F. Let the chilled pan sit at room temperature for a few minutes while the oven comes up to temperature so the center isn’t rock-hard when it goes into the oven.
Bake for 33 to 37 minutes, until the top is puffed and no longer raw-looking and the edges are just starting to brown—these visual cues are your best guide rather than color alone.
Remove the pan from the oven and set it on a wire rack to cool. When cool enough to handle, lift the slab from the pan using the parchment and place it on the rack to cool completely before slicing into squares. Store leftover bars refrigerated in an airtight container for up to 5 days.

Chill for Clean Slices

Chilling the assembled slab is the single most important step for tidy, even squares. The cold dough holds its shape in the oven so the edges don’t overbrown while the center finishes baking.

If you chill overnight, let the pan sit at room temperature for a few minutes while the oven preheats so the center isn’t rock-hard going into the oven; this helps the slab bake through evenly during the 33 to 37 minute window.

Minimum: chill at least 2 hours.
Best for slicing: chill overnight, then allow a short rest at room temperature before baking.
If your pan is very cold, give it 5 minutes out of the fridge before it goes in the oven.

Adjusting Texture

You can tweak the chew and density without changing the recipe much: more oats increases chew and heft; slightly less oats yields a softer, more tender bar.

Swapping crunchy peanut butter for creamy will add little pockets of texture and a touch more bite if you prefer contrast.

For chewier bars: increase old-fashioned rolled oats a bit (add in 1 to 2 tablespoons increments).
For softer bars: reduce oats slightly and be careful not to overbake.
To vary nut texture: use crunchy peanut butter in place of creamy for extra bite.

Tips for Success

Press evenly: Use an offset spatula or the back of a measuring cup to press the dough uniformly to the pan edges so the slab bakes at a consistent thickness.

Chill time matters: Chilling at least 2 hours (or overnight) firms the dough so the bars won’t spread and slice cleanly after baking.

Watch visual cues: Bake until the top is puffed and doesn’t look raw and the edges are just turning brown—bake time can vary by oven and pan.

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Frequently asked questions

Store leftover bars refrigerated in an airtight container for up to 5 days. Chilling helps them hold their shape and keeps the M&Ms from sweating.

Yes—wrap the cooled slab or individual squares tightly in plastic and freeze up to 2 months. Thaw in the fridge before serving.

Yes. Substitute a sunflower seed butter or another seed-based spread in place of the creamy peanut butter to make a nut-free version; note the flavor and texture will shift slightly.

Ready to bake

These slab-style Monster Cookie Bars deliver reliable, chewy texture and festive appeal—chill well, bake to the visual cues, and decorate with candy eyeballs for a fun Halloween tray.

If you plan ahead and chill the dough, the slicing and presentation are effortless, making these perfect for bake sales, packed lunches, or last-minute party platters.

Equipment

8-by-8-inch baking pan
parchment paper
stand mixer or mixing bowl
paddle attachment (optional)
rubber spatula
wire rack

Ingredients

1/2 cup unsalted butter 113g (1 stick), at room temperature
1 cup light brown sugar 210g, packed
1/2 cup creamy peanut butter 140g, at room temperature
1 teaspoon honey
1 1/2 teaspoons vanilla extract
1 large egg at room temperature
1 large egg yolk at room temperature
1 cup all-purpose flour 125g
1/4 teaspoon fine sea salt
1/4 teaspoon baking soda
1 1/2 cups old-fashioned rolled oats 150g
1/2 cup M&Ms candies 100g
1/2 cup chocolate chips 90g
candy eyeballs for topping (optional)

Instructions

Lightly butter the bottom and sides of an 8-by-8-inch baking pan, then line the pan with parchment paper so it stays in place.
In a mixing bowl or the bowl of a stand mixer fitted with the paddle attachment, beat the butter, packed light brown sugar, creamy peanut butter, honey, and vanilla until the mixture is fluffy and slightly lighter in color.
Scrape down the sides of the bowl, then add the egg and egg yolk and mix until fully incorporated.
Add the all-purpose flour, fine sea salt, and baking soda and mix on low speed until nearly incorporated. Add the rolled oats and mix briefly to distribute, then fold in the M&Ms and chocolate chips with a rubber spatula, scraping any dry bits from the bottom and sides of the bowl.
Transfer the dough to the prepared pan and use a spatula to press it evenly to the edges. Cover and chill for at least 2 hours or up to overnight.
Preheat the oven to 350 degrees F and let the chilled pan sit at room temperature briefly while the oven preheats.
Bake for 33 to 37 minutes, until the top is puffed and no longer raw-looking and the edges are just starting to brown.
Remove the pan from the oven and set it on a wire rack to cool. When cool enough, lift the slab out using the parchment and place it on the rack to cool completely before slicing into squares. Store leftover bars refrigerated in an airtight container for up to 5 days.

Notes

Chill the assembled dough at least 2 hours or overnight for the cleanest slices. Let the chilled pan warm slightly while the oven preheats so the slab bakes evenly. Store bars refrigerated in an airtight container for up to 5 days; top with candy eyeballs for a Halloween look if desired.