1. Preheat oven to 400°F. In a large bowl, beat butter and brown sugar with an electric mixer on medium speed until creamy. Beat in egg, yogurt, vanilla, and mint extract until well blended.
2. Add flour mixed with cocoa powder, baking soda, and salt and beat until well blended. Stir in chocolate chips.
3. Drop by teaspoonfuls onto greased cookie sheets. Bake 8 to 10 minutes. Let cookies cool on sheets 2 minutes, then remove to a rack and let cool completely.