In a large bowl beat butter with an electric mixer on medium to high speed for 30 seconds. Add sugar. Beat until combined, scraping bowl occasionally. Beat in egg and vanilla until combined. Beat in as much of the flour as you can with the mixer. Stir in any remaining flour. Divide dough in half.Advertisement
Add cocoa powder to one dough portion; stir until combined. Add mint extract and a few drops of green food coloring to the remaining dough portion; stir until combined. Divide each portion in half (two chocolate dough portions and two mint dough portions).
Shape each dough portion into a 9-inch rope. Loosely twist one chocolate dough rope and one mint dough rope together. Gently roll together to shape into a 9-inch roll; repeat (you will have two 9-inch rolls). Wrap each roll in plastic wrap or waxed paper. Chill about 1 hour or until dough is firm enough to slice.
Preheat oven to 350°F. Cut rolls into 1/4-inch slices. Place slices 2 inches apart on ungreased cookie sheets.
Bake for 8 to 10 minutes or until edges are firm. Transfer to a wire rack; cool completely. If desired, in a small heavy saucepan heat and stir dark chocolate over low heat until melted and smooth. Drizzle melted chocolate over cookies. Let stand until chocolate is set.
Layer cookies between sheets of waxed paper in an airtight container; cover. Store in the refrigerator for up to 3 days or freeze for up to 3 months.