Let your butter sit out until it is soft. Line cookie sheets with silicone liners, or spray with cooking spray. Preheat your oven to 350°.
Beat the butter and sugars with an electric mixer for 2-3 minutes, or until light and creamy. Beat in the egg and vanilla.
Whisk together the flour, cocoa, baking soda, and salt. (Sift if your cocoa is lumpy.)
Add the flour mixture to the wet ingredients and stir just until flour is moistened. Fold in about a cup of the mint chips.
Roll dough by tablespoons into balls and place on prepared baking sheet at least two inches apart. Bake at 350 degrees for 9-11 minutes, or until barely set in the middle.
While cookies are still hot, press a few mint chips into the top of each cookie.
After 3-5 minutes, transfer the cookies to a wire rack to cool completely.