Sift the flour, corn starch, and salt into a medium-sized mixing bowl.
Run a whisk through the dry ingredients to combine and set aside.
In a stand mixer fitted with the paddle, beat the butter and sugar on medium speed until creamy, about 5 minutes.
Turn the mixer to low and add the egg while beating, then add the vanilla and almond extract.
Add the flour mixture one cup at a time until well incorporated; the dough will pull away from the mixing bowl.
Form the dough into a ball, flatten into a disc, wrap in plastic wrap, and refrigerate for a minimum of 30 minutes.
Preheat the oven to 375°F.
Remove the dough from the refrigerator and turn it out onto a lightly floured surface.
Roll the dough to about ¼”–½” thick with a rolling pin.
Using a 2-1/2" round cookie cutter, cut as many discs as possible and place them onto a baking sheet lined with a silicone mat or parchment paper.
Gather the remaining dough, form into a ball, reroll, and cut out more cookies until all dough is used.
Bake the cookies on the baking sheet for 12 minutes, rotating the tray halfway through baking.
Remove from the oven and let the cookies cool briefly on the baking sheet before transferring to a wire rack to cool completely.
Once cookies are completely cool, decorate as desired.
Add the powdered sugar and meringue powder to a large mixing bowl attached to your stand mixer and stir briefly to combine.
Fit the mixer with the wire whip, turn to low, and slowly add the water; once the powder subsides you can increase speed to medium.
Whip the icing for about 7-10 minutes until you start to see tips forming on the whip.
Check the icing consistency by letting it fall off the wire whip: if it melts back into the bowl within 5-10 seconds, it's the correct flooding consistency.
Cover the bowl tightly with plastic wrap until ready to use.
Place a piping bag fitted with a number 3 tip into a tall glass, folding the bag edges over the glass to make filling easier.
Use a rubber band to secure the top of the bag and place a wet paper towel in the bottom of the glass to help keep the tip moist while working.
For colored icing, transfer a small amount to bowls and mix in gel food coloring to reach the desired shades.
For a white background, pipe a line of icing around the cookie edge, then flood the center and use a toothpick to spread the icing evenly.
Allow each cookie to set before moving on, or add sprinkles while the icing is still wet if using them.
Use the colored icings and a toothpick to create designs; if piping additional colors onto a white base, let the white set first before adding details.