Beat butter, white sugar, and brown sugar in a large bowl until fluffy. Stir egg, vanilla extract, and mint into the mixture until thoroughly combined. Mix flour, cocoa powder, baking soda, and salt into wet ingredients to make a smooth dough. Gently fold chocolate chips into the dough. Drop by spoonfuls onto baking sheets.
Bake in the preheated oven until cookies are set, 10 to 12 minutes.
