Or you can use the more traditional icing for thumbprint cookies that made with confectioners sugar and milk.
Mix the confectioners sugar and milk until smooth.
Color the Icing:
Place a small amount of the white icing into a piping bag or a Ziploc bag.
Mix the brown by adding a small amount of Warm Brown, Lemon Yellow and Super Red AmeriColor Food Gel and mix until you get a gingerbread brown color. Place it in a piping bag or a Ziploc bag.
Decorate:
Fill each indention with the gingerbread colors and then make a wavy line of white through the center. Let the cookies dry completely.