Recipe courtesy of Elisa Strauss.
Place cake on a clean work surface. Using a serrated knife, trim top to make level, and split cake horizontally to form 2 layers. Place a dab of buttercream in the center of a 9-by-13-inch cardboard rectangle. Center 1 cake layer on top. Using an offset spatula, frost top of cake layer with 5 cups buttercream for the filling; place remaining cake layer on top. Frost entire cake with remaining 5 cups buttercream. Chill until firm, about 30 minutes.
Meanwhile, melt chocolate morsels in a double-boiler or a medium heatproof bowl set over a pan of simmering water (do not let bowl touch water). Stir in vegetable oil until combined. Remove from heat. Dip pretzels into chocolate to coat, and transfer to a parchment paper-lined baking sheet. Let sit in a cool place until chocolate has set.
Using white glue, affix cardboard rectangle to a cutting board or piece of plywood covered with adhesive-backed paper, approximately 14-by-18 inches. Process chocolate cookies in a food processor until finely ground; you will need 2 cups of crumbs. Sprinkle top of chilled cake with crumbs. Place marzipan terracotta pot in center of cake. Sprinkle candy pebbles on top of cake to form a path. Arrange marzipan flowers and garden hose in clusters on either side of path.
Gently press pretzels into sides of cake, about 1/2-inch apart. Serve at room temperature.