Slice two even layers from each of the sponges, between ¼ and ½ in/0.6 cm and 1 cm thick.
Place a layer of the vanilla sponge on an 8-in/20-cm cardboard cake round and moisten with simple syrup. Apply an even coating of buttercream to the sponge layer, reserving 6 oz/170 g of the buttercream for décor. Top with a layer of chocolate sponge and press it down gent