Using a serrated knife, cut one of the japonais layers into 10 even wedges. Cut one layer from the sponge (between ¼ and ½ in/0.6 cm and 1 cm thick).
Place the remaining japonais layer on an 8-in/20-cm cake round and apply a thin layer of buttercream. Thoroughly moisten the sponge layer with simple syrup and place on top of the japonais layer. Apply a thin laye