1. Sift 3¾ cups powdered sugar and 3½ tablespoons meringue powder into the bowl of a stand mixer fitted with a paddle attachment. Mix on the lowest setting for 1 minute. Add 6 tablespoons warm water then 2 tablespoons light corn syrup. Increase speed to medium and continue to mix 5–6 minutes or until soft, glossy peaks form. If icing is too stiff, add 1–2 more tablespoons warm water and mix.

4. After cookies have been outlined, flood completely by filling with icing. If necessary, use a toothpick to help pull the frosting to the edges (and pop any air bubbles). Transfer finished cookies to paper towels and allow to set for at least 12 hours or overnight. Make sure icing is completely dry before moving onto the next step.

2. Remove bowl from mixer and—using a rubber spatula—stir in warm water, 1 teaspoon at a time, until icing reaches the proper consistency. To test, pick up some icing with your spatula and drop it back into the bowl. It should take 15-20 seconds to sink back into the bowl and for the surface of the icing to smooth back out. Let icing sit 20–30 minutes. Pop any bubbles that have formed on the surface with a toothpick.

5. Cover workspace with newspaper or paper towels. Gather several small bowls and some food-safe paint brushes. Add a little clear extract of choice (such as vanilla, almond or lemon) to one of the bowls and set aside. Make different shades of "paint" by dividing varying amounts of gel food coloring among several bowls; mix each with a few drops of the extract to thin out. For a darker hue, use more food coloring (and vice-versa).

3. Transfer icing to a piping bag fitted with a coupler and a small, round tip. If there is icing left over in the bowl, cover with a damp paper towel and plastic wrap until ready to use to keep it from drying out. Outline the cookies with icing by piping a thin border along the edges. Let set for a few minutes.

6. Paint colors onto the iced cookies in any pattern you’d like; overlapping, alternating shades and thinning with clear extract as desired. For a speckled design, add some "paint" or extract to your brush and tap the handle to flick spots onto cookies. If desired, add finishing touches by decorating with sprinkles, sanding sugar, edible glitter, food paint, etc. while the cookies are still wet. Let dry at least 2 hours.

Sprinkles can also be applied once paint has dried. Using any leftover icing, simply pipe small dots onto cookies to adhere.