While cookies are cooling, make Mint Buttercream Frosting. In medium bowl, mix powdered sugar and butter with spoon or electric mixer on low speed. Stir in mint extract and 1 tablespoon milk. Gradually beat in just enough remaining milk to make frosting smooth and spreadable. If frosting is too thick, beat in more milk, a few drops at a time. If frosting becomes too thin, beat in a small amount of powdered sugar.