Training a Warm Sponge for a Vertical Layer Cake

A vertical layer cake hides its spiral inside the slice rather than stacking flat layers, and the secret is in how the sponge is rolled. Bake a thin, flexible sponge only until just springy - overbaking dries it out and guarantees cracks. Turn it out hot onto a tea towel, peel the parchment, and roll it up in the towel straight away. Cooling it rolled trains the sponge to hold a curl, so when you unroll, fill, and re-roll it around the buttercream it coils smoothly without splitting. Keep the buttercream layer thin: too much filling forces the sponge to stretch and tear at the outer edge. Once filled, the roll is sliced into pinwheels and stood upright, or cut into strips and coiled into a tall round, so the spiral runs vertically through the finished cake. Chill before the final coat to firm everything up.