Preheat oven to 350º.
Coat bottom of an 8" square baking pan with cooking spray.
Combine butter and mints in a 2-cup glass measure;
microwave at high 30 seconds or until soft.
Stir until smooth, and set aside.
Lightly spoon flour into a dry measuring cup,
and level with a knife.
Combine flour, soda, and salt in a bowl.
Combine sugar, cocoa, egg, and egg white in a large bowl;
beat at medium speed of a mixer until well-blended.
Add mint mixture; beat well. Add flour mixture;
beat at low speed just until blended.
Pour batter into prepared pan.
Bake at 350º for 20 minutes or until a wooden pick
inserted in center comes out clean;
cool completely on a wire rack.