Trim the vanilla sponge and slice lengthwise into 3 strips, 3 by 16 in/8 by 41 cm each.
Moisten one of the sponge strips with one-third of the syrup. Spread with half of the jam, then with 3 oz/90 g of the buttercream. Place a second strip on top of the buttercream. Moisten with one-third of the syrup and spread with the remaining jam, then with 3 oz/90 g of th