Sponge cake and whisked batters

Why

A classic sponge needs no baking powder: it is raised by the air whisked into the eggs and sugar, which expands in the oven heat before the proteins set and firm up the foam. That is why eggs and sugar are whisked into a thick, pale foam that falls from the whisk in a slow ribbon and briefly leaves a trail on the surface; with an electric mixer this usually takes a few minutes, but the look matters, not the clock. Flour, fat and vigorous stirring all destroy bubbles, so the flour is folded in gently with a spatula in portions, and melted butter is first mixed with a little of the batter – otherwise it sinks and leaves a dense, rubbery layer under the sponge. The foam is unstable and loses air while it waits, so the batter goes straight into a preheated oven, and until the centre has set, opening the oven can easily make it collapse: the temperature drops and the bubbles shrink before the proteins can set them. The same goes for whisked batters in which baking powder only helps, such as a bundt cake where the eggs and sugar are whisked to a foam.

How to do it

• Let the eggs come to room temperature first; cold eggs trap less air.
• Whisk the eggs and sugar until the foam falls from the whisk in a thick ribbon that briefly holds on the surface.
• Sift the flour over the foam in two to three portions and fold it in from the bottom up with a spatula, just until no dry patches remain.
• Mix lukewarm melted butter with part of the batter first, then fold that into the rest.
• Line only the base of the tin with paper, put the batter straight into a preheated oven and don't open it during the first half of baking.
• A sponge that is done has a golden top that springs back when pressed lightly in the centre.

If it goes wrong

A sunken or flat sponge can't be made airy again, but don't throw it away: cut it up for layered slices, tiramisu or a dessert in a glass, where cream soaks into it. Trim off a rubbery layer at the bottom. Next time whisk the eggs longer, fold in the flour more gently and don't let the batter wait.