Instructions

1. Sift 3 1/2 cups (1 pound) powdered sugar through a fine-mesh strainer into a large bowl. If any lumps are left in the strainer, use your fingers to break them up and push them through the strainer.
2. Place 4 large pasteurized egg whites or 1/2 cup liquid egg whites in the bowl of a stand mixer fitted with a whisk attachment. (Alternatively, use an electric hand mixer and large bowl.)
3. Whip on medium-high speed until foamy, doubled in volume, and opaque, about 2 minutes.
4. Turn the mixer off and add half of the powdered sugar. Mix on low speed until the powdered sugar is completely dissolved and no large lumps remain, 1 to 2 minutes. Add the remaining powdered sugar and continue mixing until completely dissolved, 1 to 2 minutes more.
5. Increase the mixer speed to high and whip until the icing is completely smooth, glossy, and slightly thickened, 4 to 5 minutes. The icing should hold a soft peak and slowly run off of the whisk attachment. If the icing is too loose, add more powdered sugar, 1/4 cup at a time, and continue mixing until completely incorporated.
6. Decrease the mixer speed to low and add 1 teaspoon extract and a few drops of food coloring, if desired. Mix until the icing is smooth and evenly colored, 1 to 2 minutes.
7. The icing can be used as-is or thinned out with water to achieve a pourable consistency for flooding cookies. Add water in 1 teaspoon increments, stirring between each, until desired consistency is reached. If not using immediately, press a sheet of plastic wrap directly on the surface of the frosting to keep it from drying out until ready to use.
8. Decorate cookies with the icing and let completely dry before touching them, at least 4 hours.