Slice three even layers from the sponge, between ¼ and ½ in/0.6 cm and 1 cm thick. Place one layer on an 8-in/20-cm cardboard cake round and moisten with simple syrup.
Using a pastry bag fitted with a #5 plain tip, pipe a ring of whipped ganache around the outer edge of the cake layer; then, leaving a gap of approximately 2 in/5 cm, pipe a circle of the ganache