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 round and moisten with simple syrup. Spread the jam in a thin, even layer on the first layer of sponge, then apply an even coating of buttercream. Top with a second sponge layer and press down gently. Moisten with syrup, apply an even coating of b