We swirled chocolate and white-chocolate ice creams together to serve with this cake. For the recipes, go to marthastewart.com/marbled-ice-cream.
Trim each cake to an even thickness. To fill each tier (amounts below), set 1 cake on corresponding foam board, securing with a dab of buttercream. Using an offset spatula, spread ganache over top; refrigerate until hardened. Spread chocolate buttercream over ganache; refrigerate until firm. Spread white-chocolate buttercream over chocolate buttercream. Stack second cake on top, and fill as before. Top with the third cake. Coat tier with a thin layer of white-chocolate buttercream; refrigerate at least 1 hour.
Spread tops and sides of chilled tiers with a smooth 1/4-inch-thick layer of white-chocolate buttercream. Refrigerate 30 minutes. Trim dowels to the heights of the three largest tiers, and insert in tiers.
Place white chocolate in a heatproof bowl over a pan of simmering water, stirring until melted. For 5-inch tier, cut a marbled transfer sheet to the height and circumference of tier (you may need several sheets; cut them to meet without overlap). Spread a thin layer of white chocolate over the marbling to the corners and edges, smoothing with an offset spatula. Let stand a few seconds to thicken slightly. Carefully lift sheet by the corners, and wrap snugly around the chilled tier, with acetate facing out. Cut plain acetate into a round to fit top of 5-inch tier. Spread a thin layer of white chocolate to the edges, and smooth. Let stand a few seconds to thicken slightly. Carefully place on top of cake with acetate facing up. Refrigerate until chocolate hardens. Carefully peel off acetate. Using a sharp knife, carefully trim edges. Refrigerate until ready to assemble. Repeat with remaining tiers.
Place a cake mat on a cake stand. Stack tiers, smearing some royal icing between tiers and pressing gently to secure; place the 6-inch riser on the 9-inch tier and the 4-inch riser on the 7-inch tier. Refrigerate until ready to display and serve. Filling and Coating Amounts for Tiers:5-inch: 1/4 cup ganache per layer (1/2 cup total), 1/4 cup chocolate buttercream per layer (1/2 cup), 1/4 cup white-chocolate buttercream per layer (1/2 cup), plus 1 cup for coating the tier.7-inch: 1/4 cup plus 2 tablespoons ganache per layer (3/4 cup total), 1/2 cup chocolate buttercream per layer (1 cup), 1/2 cup white-chocolate buttercream per layer (1 cup), plus 1 1/2 cups for coating the tier.9-inch: 3/4 cup plus 2 tablespoons ganache per layer (1 3/4 cups total), 1 cup chocolate buttercream per layer (2 cups), 1 cup white-chocolate buttercream per layer (2 cups), plus 1 1/2 cups for coating the tier. 11-inch: 1 cup ganache per layer (2 cups total), 1 1/3 cups chocolate buttercream per layer (2 2/3 cups), 1 1/3 cups white-chocolate buttercream per layer (2 2/3 cups), plus 2 cups for coating the tier.