Line a 15x10x1-inch baking pan with foil, extending the foil over the edges of the pan. Butter the foil; set pan aside.
In a medium saucepan, melt 3/4 cup butter over low heat, stirring occasionally. Remove the saucepan from the heat. Stir in granulated sugar, brown sugar and cocoa powder till combined.
Add eggs and vanilla. Using a wooden spoon, beat lightly until just combined.
In a small bowl, combine flour, baking powder, baking soda, and salt. Add flour mixture and milk alternately to chocolate mixture, beating with the wooden spoon after each addition. Stir in nuts.
Spread half of the batter into the prepared pan. Sprinkle mints evenly over the batter. Pour remaining batter evenly over top. Bake in a 350 degree F oven for 18 to 20 minutes or until a wooden toothpick inserted near the center of brownie comes out clean (do not insert into a mint patty piece) . Cool completely in pan on a wire rack.
Frost with Chocolate Frosting. Sprinkle with decorating sugar, if you like. Use foil to carefully lift brownie out of pan. Cut brownie into 24, 2-1/2-inch squares, then cut each square diagonally to make triangles. Makes 48 bars.
In a medium mixing bowl, combine butter, cocoa powder, and milk. Add powdered sugar and beat with an electric mixer till the frosting is spreadable. If needed, add additional milk, 1 tablespoon at a time, to reach desired consistency. Makes about 1-3/4 cups.