Preheat oven to 350°F (175°C) and line an 8×8-inch pan with parchment paper
Melt butter and chocolate together until smooth, then cool slightly
Whisk in granulated sugar and brown sugar until glossy
Add eggs one at a time, mixing well after each
Stir in vanilla extract and peppermint extract
Gently fold in flour, cocoa powder, and salt just until combined
Fold in mint chocolate chips evenly
Spread batter evenly into prepared pan
Bake for 30–35 minutes, until edges are set and center is slightly soft
Cool completely before slicing