In a large mixing bowl beat butter with an electric mixer on medium to high speed about 30 seconds or until softened. Add about half of the flour, the powdered sugar, cocoa powder, and mint extract. Beat on low speed until combined, then on medium speed until blended. Stir in the remaining flour. Stir in the chocolate pieces. If necessary, chill dough for 1 to 2 hours for easier handling.
On a lightly greased cookie sheet pat the dough into a 9-inch circle. Using your fingers, press to make a scalloped edge. With a fork, prick dough deeply to make 16 wedges.
Bake in a 300 degree F oven about 25 minutes or until edges are firm to the touch and center is set. Let cool for 2 minutes on the baking sheet. With a long, sharp knife, cut along the perforations into wedges. Carefully transfer to a wire rack; cool.
Drizzle with melted candy coating, if desired. Sprinkle with crushed candies before the coating dries, if desired.