Chocolate sponge

A chocolate sponge is a vanilla sponge in which part of the flour has been swapped for cocoa. It has to carry a filling without crumbling. The usual complaint is a dry, slightly bitter layer, and the cause is cocoa itself: it soaks up liquid like flour does, but has no gluten to hold the structure.

Ingredients and what each one does

• Eggs whisk into the foam that lifts the sponge.
• Sugar stabilises the foam, keeps the crumb moist and softens the bitterness of cocoa.
• Flour is the frame.
• Cocoa gives flavour and colour. It replaces part of the flour instead of sitting on top of the usual amount, because extra dry powder means a drier layer. Use fresh cocoa; old cocoa turns bitter.

How to make it

1. Whisk room-temperature eggs with the sugar until thick, pale and ribbon-like, usually 8–12 minutes.
2. Sift the flour and cocoa together. Cocoa clumps, and sifting spreads it evenly so you don’t have to stir more later.
3. Fold the dry mix in over several additions with a spatula, only until the streaks of dry powder disappear. Small lumps don’t matter; long mixing deflates the foam and builds gluten.
4. Line the tin with parchment and fill no more than two-thirds. Dusting the sides with cocoa instead of flour avoids a pale coating on the crust.
5. Bake 25–30 minutes at 170 °C fan without opening the door early. Done means a clean skewer and a springy top; leave it a few more minutes in the switched-off oven.
6. After 10–15 minutes, turn it onto a rack. Don’t wrap it warm. Chilled for a few hours, the layer cuts more evenly, especially with a wire.

What can go wrong

• Dry layer. Overbaked, or too much cocoa relative to flour and eggs. The second one is a recipe problem, not a baking problem, so always soak a chocolate sponge.
• Bitter aftertaste. Stale or very strong cocoa. Don’t add sugar to cover it; buy fresher cocoa.
• Sunken. The foam collapsed from overmixing or from opening the oven door too early.
• Crumbly layer. Too much cocoa for the amount of flour and egg, so there isn’t enough structure.

Substitutions

Dutch-process (alkalised) cocoa is darker and milder; natural cocoa is more acidic and lighter in colour. A whisked sponge has no soda that needs acid, so either works, but the colour and taste will differ. If you move to a baking soda and oil batter, that’s a different recipe: soda needs the acidity of natural cocoa.

Storage

Wrapped, a day at room temperature, 3–4 days in the fridge, a month in the freezer. Once soaked and assembled, the cake tastes better on day two.