In a large mixing bowl beat butter (or margarine) with an electric mixer on medium speed about 30 seconds or until softened. Add about half of the flour. Add cocoa powder, sugar, egg yolk, vanilla, baking powder and mint extract. Beat until thoroughly combined. Beat or stir in remaining flour. Do not chill dough. Pack dough into a cookie press. Force dough through the press onto an ungreased cookie sheet If desired, decorate with candies, fruits, nuts, or colored sugars. Bake in a 375F oven for 8 to 10 minutes or until edges are firm but not browned. Remove cookies from the cookie sheet; cool on a rack.