Preheat oven to 325°.
To prepare the brownies, melt 6 tablespoons butter in a medium saucepan over medium heat. Stir in cocoa and milk; cook 1 minute, stirring constantly. Add brown sugar, granulated sugar, and vanilla; cook mixture 1 minute, stirring constantly. (Mixture will be granular.) Remove mixture from heat; cool 5 minutes. Add egg whites, 1 at a time, stirring well after each addition. Lightly spoon flour into a dry measuring cup, and level with a knife. Combine the flour, baking powder, and salt, stirring well with a whisk. Add the flour mixture to the cocoa mixture, stirring until well-blended. Pour the batter into a 9-inch square baking pan coated with cooking spray. Bake at 325° for 25 minutes or until brownie barely springs back when touched lightly in center; cool completely on a wire rack.
To prepare icing, combine powdered sugar, 1 tablespoon butter, and liqueur. Beat with a mixer at medium speed until smooth. Spread the icing evenly over brownie layer; refrigerate 30 minutes or until icing is set.
To prepare glaze, place chocolate in a small microwave-safe dish. Microwave at high for 45 seconds; stir in 1 tablespoon butter. Microwave at high for 20 seconds. Stir until smooth. Drizzle the glaze over icing; refrigerate for 30 minutes or until the glaze is set.