Walnut Cake

Eggs are doing two jobs at once here and that's worth separating in your head. The yolks, beaten with sugar and vanilla until pale and thick, are an emulsifying and flavour-carrying base — that's where the fat from the butter gets absorbed smoothly instead of sitting in greasy pools. The whites are structural, the scaffolding that gives the sponge its lift. Use eggs at room temperature if you can — cold whites whisk up more slowly and hold less air, which is a small thing but it stacks with the folding technique above. There's no substitute for egg here that behaves the same way; this isn't a cake you can make eggless and expect the same lift.

Butter softened at room temperature, not melted — melted butter is liquid fat and behaves completely differently when beaten into a batter, it won't trap air the same way and the crumb goes denser and greasier. "Softened" means you can press a thumbprint into it without a fight.

The ground walnuts aren't a flavouring sprinkled in, they're replacing roughly a third of the flour you'd expect in a sponge this size. Any finely ground nut with reasonably neutral oil would behave similarly — almonds, hazelnuts — but walnuts carry a slight bitterness and tannic edge that plays nicely against the sweetness of the glaze, which is the actual reason this recipe calls for them rather than tradition for tradition's sake. Grind them yourself if you can get whole walnuts cheaply; pre-ground walnut meal from a shop is often older and the oils in it can turn rancid and musty, which you will absolutely taste.

Vanilla pod twice in this recipe, once for the sponge and once for the cream, is there because vanilla extract and vanilla seeds taste genuinely different — the seeds carry little flecks of actual flavour and a rounder, less alcoholic aroma. If pods are expensive where you are (and in the UK they often are), a good vanilla paste does nearly the same job; a teaspoon of ordinary extract is the fallback, just added at the same point rather than scraped.

The whipping cream needs to be cold from the fridge right up until the moment you whip it — not "fairly cool," cold — because warm cream fat doesn't trap air properly and will split into butter and liquid before it reaches stiff peaks if you're not careful. 800ml is a lot of cream for one sponge, and that volume is the point: this cake is cream-forward, the sponge is really a frame for it. Dark chocolate roughly chopped rather than melted gives you bite and little pockets of bitterness through the cream instead of a uniform chocolate flavour — melt it and fold it in and you'd get a denser, heavier filling, not better, just different.

Rum brushed onto the layers isn't decoration, it's moisture insurance: a walnut sponge, being drier than a plain flour sponge because of the nut flour and the baking time, needs that liquid to stay pleasant three layers deep instead of chalky by the third bite. If you don't want alcohol, a light sugar syrup with a dash of vanilla does the same structural job, just without the warmth rum adds.

The coffee glaze — icing sugar beaten with coffee rather than water or milk — is an old trick for cutting the flatness of icing sugar's sweetness with bitterness, the same logic as the dark chocolate inside. Instant coffee dissolved strong works fine; you're not making an espresso here, you're making a flavoured liquid to loosen sugar into a spreadable paste, so brew it stronger than you'd drink it.

One honest note about equipment: a 20cm springform tin is specified because releasing a delicate sponge from a solid tin without one is how sponges get wrecked at the final hurdle. If you don't have one, a loose-bottomed cake tin is the next best thing — a solid tin with no release mechanism at all is asking for trouble on a cake with this much structure to protect.