Prepare cake batter according to package directions, substituting cold brewed coffee for water. Spoon into 3 wax paper-lined 9-inch round cakepans.
Bake at 350° for 20 to 25 minutes. Cool in pans on wire racks.
Invert 1 layer onto a serving plate; remove wax paper, and spread with half of White Chocolate Mousse. Repeat procedure with remaining cakelayers and mousse, ending with a cake layer. Cover and chill at least4 hours.
Spread half of Coffee Buttercream Frosting on top and sides of cake. Pipe border around bottom using a star tip; pipe large dollops around top of cake. Sift ground cinnamon over top of cake. (Freeze up to 1 month; thaw in refrigerator 8 hours.) Garnish, if desired.
Prep: 20 min., Bake: 25 min., Chill: 4 hrs.