Sand Tarts (Mimi's Christmas cookies)

Eggs, food coloring, and/or sprinkles for decorating
 * 1/2 Cup butter
 * 1 Cup granulated sugar
 * 1 egg
 * 1 tsp vanilla extract
 * 1 3/4 cups flour
 * 2 tsp baking powder

Cream butter and sugar, add egg and vanilla. Add flour and baking powder. Chill dough. Brush with egg white and add sprinkles, or paint with egg yolk paint. Back at 350 F for 5-8 minutes