Prepare and bake 1 (21-oz.) package fudge brownie mix according to directions for a 13- x 9-inch pan. Let cool in pan on a wire rack for 45 minutes.
Stir together 1/2 cup butter, 1/3 cup milk, and 5 Tbsp. unsweetened cocoa over low heat, stirring constantly, 5 minutes or until butter melts and mixture is blended. Remove from heat, and beat in 1 (16-oz.) package powdered sugar at low speed with an electric mixer until smooth; spread frosting evenly over prepared brownies. Let stand 45 minutes or until frosting is firm. Cut into 12 equal squares.
Tint 1/2 (16-oz.) container ready-to-spread vanilla frosting with desired shade of food coloring gel. Spoon frosting mixture into a small zip-top plastic freezer bag. Snip a hole in 1 corner of the bag. Pipe desired monogram on each brownie.
* You can find small letter-shaped cookie cutters at crafts stores to use as templates. Simply imprint letter on iced surface; then pipe over the imprint.
* If you don't have time to monogram, pick up a box of alphabet-shaped cookies to use as a topping.