1. In a bowl beat butter on medium to high speed for 30 seconds. Add brown sugar, granulated sugar, egg, and vanilla; beat until combined. In another bowl combine baking soda, salt, cocoa, instant mint cocoa packets, and flour; sift all these dry ingredients until well combined. Beat in as much of the flour mixture as you can with the mixer. With a wooden spoon, stir in remaining flour mixture. Stir in mint chocolate chips/or mint pieces. Refrigerate about 6 hours.
2. Roll into 1 inch balls. Place 2-3 inches apart on an un-greased cookie sheet. Dip the bottom of a drinking glass in sugar and flatten the dough balls to about 3/8 inches. The sugar on the bottom of the glass prevents the dough from sticking while flattening. Bake in a 375 degree oven for 10 minutes. Cool on wire rack.