Preheat oven to 350°F. Grease the bottom of a 9x1 1/2-inch round cake pan. Line bottom of pan with parchment paper. Grease parchment; set aside.
Prepare brownie mix according to package directions. Pour into prepared pan. Bake according to the package directions for a 9x9-inch square pan or until a wooden toothpick inserted near the center comes out clean. Cool completely on wire rack. Remove brownie from pan. Place cooled brownie on a baking sheet.
In a small saucepan, bring cream just to boiling over medium-high heat. Remove from heat. Add chocolate (do not stir). Let stand for 5 minutes. Stir until smooth. Chill for about 5 minutes or until it reaches a spreading consistency. Spread chocolate over top of brownies in an even layer. Chill for 30 minutes.
Meanwhile, prepare Marshmallow Coffee Frosting. Pipe or spread Marshmallow Coffee Frosting onto top of cake. Chill for 15 minutes.
Preheat oven to 450°F. Bake about 5 minutes or until lightly browned. Cool completely on a wire rack. If desired, dust with cocoa powder.
In the top of a 2-quart double boiler combine sugar, the cold water, egg whites, and cream of tartar. Beat with an electric mixer on low speed for 30 seconds. Place the double boiler top over boiling water (upper pan should not touch water). Cook, beating constantly with the mixer on high speed, for 10 to 13 minutes or until an instant-read thermometer registers 160°F when inserted in mixture. Remove pan from heat. Add vanilla. Beat about 1 minute or until frosting is fluffy and holds soft peaks (tips curl).