Preheat oven to 350 degrees F.
Coat bottom of an 8-inch square baking pan with cooking spray.
Combine butter and mints in a 2-cup glass measuring cup; microwave on 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 350 degrees F for 20 minutes or until a wooden pick inserted in center comes out clean; cool completely on a wire rack.
Recipe reprinted by permission of Cooking Light magazine. All rights reserved.