Prepare cake batter and bake in two 9-inch round baking pans as directed on package. Cool completely. Meanwhile, beat sour cream and powdered sugar in large bowl with wire whisk until well blended. Gently stir in whipped topping. Cover and refrigerate until ready to use.
Cut one cake layer in half as shown in Diagram 1 of Sombrero Cake; cut remaining cake layer as shown in Diagram
Arrange both Brim pieces, the Top piece and Base of Brim piece on cake board or large platter as shown in Diagram 3 to resemble sombrero.
Cut Band and Mustache pieces horizontally in half to make two pieces of each.
Place both Band pieces on cake as shown. Reserve both Moustache pieces for later use.
Spread 1-1/4 cups of the frosting evenly onto all cake pieces except mustache. Refrigerate or freeze until frosting is firm. Meanwhile, melt chocolate as directed on package.
Spread cake with remaining frosting.
Place Mustache pieces at bottom of sombrero; cover with some of the melted chocolate. Decorate cake with remaining chocolate, raspberries and colored sugar. Store in refrigerator.
