4 cups powdered sugar, sifted
2 tbsp. meringue powder
5 tbsp. water
Combine all ingredients in the bowl of a stand mixer fitted with the paddle attachment. Mix on low speed until the sheen has disappeared and the icing has a matte appearance (about 7-10 minutes). Transfer the contents of the mixing bowl to an air-tight container. This will be the stiffest consistency of the icing, and at this point it is still too stiff to use for decorating. Add water a very small amount at a time and stir by hand until fully incorporated. Continue until the icing has reached a consistency appropriate for piping. (Remember, if you are having any difficulty piping, it is still too thick. Add a little more liquid and try again.) Using a pastry bag, pipe around the edges of each cookie. Let stand so the icing will set. Make sure to keep the leftover icing covered at all times when not in use so that it does not begin to harden.
Once all the cookies have been edged, transfer some of the remaining icing to a separate air-tight container. Thin out by incorporating a small amount of water at a time, until the icing drips off the spoon easily when lifted and then smooths in with that still in the bowl. If you go too far and the icing is too thin, add more sifted powdered sugar to thicken it again. Once the icing has reached the desired consistency, transfer it to a squeeze bottle (or a plastic bag with a hole in one corner), and flood the area surrounded by the piping on each cookie. If it does not completely spread to the edges, use a toothpick to help it along. Allow to set.
Use the remaining thicker icing for piping decoration as desired. Gel icing color is best as it does not add a significant amount of liquid. Liquid food coloring can be used as well – add powdered sugar as needed to compensate for any thinning that occurs.
--Adapted from Katie of Good Things Catered