Prepare and bake cake according to package directions, using two greased 9-in. round baking pans. Cool for 10 minutes before removing from pans to wire racks to cool completely.
Spread frosting between layers and over top and sides of cake. Separate candy bars; lightly press into sides of cake. Arrange berries on top; brush with preserves. Garnish with mint if desired.
How-To: To keep the cake stand clean while frosting, line the edge with 3-in. strips of waxed paper. Center the first cake layer over the strips. When finished frosting, carefully remove strips one piece at a time. See cake stand»