1. Beat butter, sugar, egg and vanilla in large bowl until well blended. Stir together flour, cocoa, baking powder and salt; gradually add to butter mixture, beating until well blended. Divide dough in half; place each half on separate sheet of wax paper.
2. Shape each portion into log, about 7 inches long. Wrap each log in wax paper or plastic wrap. Refrigerate until firm, at least 8 hours.
3. Heat oven to 325°F. Cut logs into 1/4- inch-thick slices. Place on ungreased cookie sheet.
4. Bake 8 to 10 minutes or until set. Cool slightly; remove from cookie sheet to wire rack. Cool completely.
5. Prepare frosting, if desired; spread over top of cookies. Makes about 54 cookies.
1/4 cup (1/2 stick) butter, softened
1-1/2 cups powdered sugar
1 to 2 tablespoons milk
1/2 teaspoon vanilla extract
Yellow food color
Gradually add powdered sugar and milk to butter, beating to desired consistency. Stir in vanilla and food color. About 1 cup frosting.