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 half into log with squared sides, about 7 inches long and 2-1/4 inches wide. 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 3/8-inch thick slices. Place onto ungreased cookie sheet.
4. Bake 15 minutes or until set. Cool slightly; remove from cookie sheet to wire rack. Cool completely. Remove wrappers from chocolate pieces.
5. Prepare QUICK COOKIE GLAZE; drizzle cookies. Immediately place a chocolate piece on each cookie. Makes about 36 cookies.
QUICK COOKIE GLAZE: Stir together 3/4 cup powdered sugar, 3 to 4 teaspoons milk, 1/4 teaspoon vanilla extract and 3 or 4 drops green food color, if desired, in small bowl until of drizzling consistency.