How to Make It
Prepare Cookies: Beat butter and granulated sugar at medium speed with an electric mixer 5 to 7 minutes or until light and fluffy. Add egg and vanilla; beat 1 minute.
Sift together flour and next 2 ingredients. Add to butter mixture all at once, and beat at medium speed 1 minute or until combined. Flatten dough into a 1-inch-thick disk, and wrap in plastic wrap. Chill 2 hours.
Preheat oven to 375°, with oven racks in 2 middle positions. Place dough on a lightly floured surface; roll to 1/4-inch thickness. (If dough gets too soft to handle, chill 5 to 10 minutes.)
Cut dough with desired cutters, and place 1/2 inch apart on parchment paper-lined baking sheets. Reroll scraps as needed. Chill cut cookies 30 minutes.
Bake cookies, 2 sheets at a time, at 375° for 9 minutes. Rotate pans front to back, and top rack to bottom rack. Bake 2 to 4 more minutes or until slightly golden around edges. Remove cookies to wire racks, and cool completely (about 15 minutes).
Meanwhile, prepare Icing: Beat powdered sugar, egg whites, and 2 tsp. water at low speed 2 minutes. (If necessary, add up to 1 Tbsp. water, 1 tsp. at a time, to reach desired consistency.) Divide icing, and tint with food coloring, if desired. Cover icing with plastic wrap or a damp towel when not in use.
Spread a thin layer of icing over each cookie. For a swirled effect, immediately dot cookie with another color of icing, and blend with a wooden pick. Let base layer of icing dry 10 minutes before piping details. Let finished cookies dry 15 minutes before serving.