Run a knife around edges of sponge cake to release it from pan. Reverse cake onto a piece of waxed paper and peel off the parchment paper liner. Brush sponge all over with liqueur. Spread evenly with about 1½ cups of Chocolate Buttercream. Cover and refrigerate remaining buttercream until needed.
Cut chocolate-covered sponge into 6 equal pieces, each about 4 in