How to Make It
Prepare Brownies: Preheat oven to 350°. Line bottom and sides of a 13- x 9-inch pan with aluminum foil, allowing 2 to 3 inches to extend over sides; lightly grease foil with cooking spray. Microwave butter and chopped bittersweet chocolate in a large microwave-safe bowl at HIGH 1 1/2 to 2 minutes or just until melted and smooth, stirring every 30 seconds. Whisk in sugar and vanilla. Add eggs, 1 at a time, whisking just until blended after each addition. Stir together flour, baking powder, and salt in a small bowl. Whisk flour mixture into chocolate mixture until blended. Pour mixture into prepared pan.
Bake at 350° for 18 to 20 minutes or until a wooden pick inserted in center comes out with a few moist crumbs. Cool completely on a wire rack (about 1 hour). Lift brownies from pan, using foil sides as handles. Cut 8 circles, using a 3-inch round cutter. Reserve scraps for another use.
Prepare Milk Chocolate Mousse: Microwave milk chocolate morsels and peanut butter in a small microwave-safe glass bowl at MEDIUM (50% power) 1 1/2 to 2 minutes or until melted and smooth, stirring every 30 seconds. Cool 5 minutes.
Beat 1 cup heavy cream at medium speed with an electric mixer until soft peaks form; fold cream into milk chocolate mixture. Chill while making White Chocolate Mousse.
Prepare White Chocolate Mousse: Microwave white chocolate morsels and 1/4 cup cream in a small microwave-safe glass bowl at MEDIUM (50% power) 1 1/2 to 2 minutes or until melted and smooth, stirring every 30 seconds. Cool 5 minutes.
Beat remaining 1 cup cream at medium speed with an electric mixer until soft peaks form; fold into white chocolate mixture. Chill while preparing foil molds for Step
Assemble stacks: Wash and dry paper clips. Cut heavy-duty aluminum foil into 8 (6- x 10-inch) pieces. Fold each piece in half to form a 3- x 10-inch strip. Wrap each strip around a 3-inch-diameter can. (This helps create a smooth curve.) Wrap 1 curved foil strip around each brownie; secure with a large paper clip. Immediately spoon Milk Chocolate Mousse into a zip-top plastic freezer bag. (Do not seal.) Snip 1 corner of bag to make a small hole (about 1/2 inch). Pipe mousse onto brownies, dividing mixture evenly. Use a small spoon to level gently. Repeat procedure with White Chocolate Mousse. Chill 2 hours; remove foil to serve.