Prepare 2 1/2 recipes Chocolate Cake, 1 recipe at a time, and bake layers according to directions.
Wrap each layer in plastic wrap; freeze 2 hours.
Cut domed top off each layer using a serrated knife, if necessary.
Cut 1 (13- x 9-inch) and 1 (9- x 5-inch) rectangle from sturdy cardboard; cover with aluminum foil.
Spread a small amount of Chocolate Ganache Frosting on 1 side of 13- x 9-inch cardboard rectangle; place 1 (13- x 9-inch) cake layer on top. Spread 2 1/2 cups Pecan Filling over cake layer, leaving a 1-inch border. Top cake with the remaining 13- x 9-inch layer.
Cut 4 wooden skewers to height of 13- x 9-inch tier; insert skewers vertically into cake tier, evenly spaced and level with top of tier, about 3 inches from sides.
Assemble 2 (9- x 5-inch) layers on cardboard as directed above, using remaining 1/2 cup Pecan Filling and omitting wooden skewers.
Spread top and sides of both tiers with frosting, smoothing with a wet metal spatula.
Chill tiers up to 2 days, or freeze up to 2 months.
Position 9- x 5-inch tier in the center of 13- x 9-inch tier; place cake on stand.
Insert a coupler into a decorating bag. Fit coupler with metal tip #32; fill with frosting. Pipe a shell border on top and bottom edges of each tier. Pipe vines onto tiers using metal tip # Pipe leaves onto vines using metal tip #
Attach Chocolate-Covered Twigs to cake with a small amount of frosting. Arrange leaves and blackberries on tops and sides of tiers.
You can chill the cake tiers for up to 2 days, or freeze up to 2 months. Be sure to remove the blackberry leaves and twigs before eating cake.