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.