Sacher sponge

A Sacher sponge is denser than an ordinary chocolate cake because it uses melted dark chocolate and butter rather than cocoa. That gives moisture and a mellow flavour, but the egg foam carries more weight. The usual failure is a sunken middle: the foam deflated while the chocolate mixture was being folded in.

Ingredients and what each one does

• Dark chocolate gives flavour, colour and structure, as its cocoa butter sets when cool.
• Butter keeps the crumb tender.
• Eggs do the rising. Whipped whites make it airy, yolks give colour and moisture.
• Sugar stabilises the whites and keeps the crumb moist.
• A little flour only holds the structure together. Too much makes the cake dry and rubbery.

How to make it

1. Eggs, butter and chocolate at room temperature or warm. Cold butter seizes into lumps in chocolate.
2. Melt the chocolate with the butter over a water bath, then cool to just warm, about 35 to 40 °C. Hot chocolate scrambles the yolks.
3. Beat the yolks with part of the sugar until pale, then stir in the chocolate mixture.
4. Whip the whites with the rest of the sugar to stiff peaks. Sign: the sugar is dissolved and the foam does not slide off the whisk.
5. Stir a third of the foam in firmly to loosen the base, then fold in the rest with a spatula, sifting in the flour in parts. Stop when no white streaks remain.
6. Fill a greased, lined tin no more than two thirds.
7. Bake at about 165 °C with a fan for roughly 40 minutes. Sign: a skewer comes out with a few moist crumbs, not wet batter. Chocolate sponge dries out fast, so do not overbake.
8. Rest 10 to 15 minutes in the tin, then cool on a rack. Do not wrap it warm, condensation forms. Chilled for several hours it cuts much more cleanly.

What can go wrong

• Sunk in the middle. The foam deflated: the chocolate mixture was too hot, or folding went on too long. Opening the oven early can do it too.
• Dry and crumbly. Overbaked, or too much flour. Syrup helps but does not fix the cause.
• Dense streak at the bottom. The flour settled. Fold from the bottom up.

Substitutions

Milk chocolate makes the cake sweeter and softer but weaker in structure. Cocoa instead of chocolate needs extra fat, so it becomes a different cake.

Storage

Wrapped in film in the fridge for 2 to 3 days. Chill it for a few hours before assembling.