PREPARE cake batter and bake in 13x9-inch pan as directed on package, beating the dry pudding mix into the batter before pouring into prepared pan. Cool cake 10 min.; invert onto wire rack. Remove pan; cool cake completely.
MELT chocolate as directed on package; cool 5 min. Pour into large bowl. Add sugar, butter and vanilla; beat with mixer on low speed until well blended. Gradually beat in milk until well blended and of spreading consistency.
TRANSFER cake to cutting board; cut into pieces as shown in Diagram 1. Arrange cake pieces on tray or platter as shown in Diagram 2, securing pieces together with some of the frosting.
FROST cake with remaining frosting. Decorate with remaining ingredients as shown in photo.