Chocolate Birthday Cake

Start with the eggs, because the whole sponge depends on separating what they do. Yolks carry fat and richness and help emulsify the melted chocolate-butter mixture into the batter smoothly — that's why they go in first, beaten pale with the sugar, so the sugar partly dissolves and the mixture lightens in colour, a sign it's actually aerated and not just stirred. Whites, beaten separately to a stiff foam, are pure lift. Folding them in last and gently is non-negotiable: any vigorous mixing here deflates the one source of rise you have, since 8g of baking powder on 120g flour is doing modest work, not heavy lifting.

Dark chocolate and butter melted together is a classic pairing because butter is mostly fat with a little water, and that water is what can make melted chocolate seize if you're not careful — doing it over a bain-marie or gently in the microwave, in short bursts, avoids that. Any dark chocolate with 60-70% cocoa works; anything much higher and the cake turns bitter rather than rich, since there's no extra sugar built in to compensate. Letting this mixture cool completely before it meets the egg yolks matters for a plain reason: hot chocolate-butter poured onto yolks will start cooking them, giving you flecks of scrambled egg in your cake batter rather than a smooth one.

The milk chocolate mousse (300g milk chocolate, 500ml cream used two ways) works through a basic ganache-into-whipped-cream trick. You make a ganache first with part of the cream brought to the boil — boiling, not simmering, because you need enough heat to melt 300g of chocolate through fully, not just soften the surface — then you fold that cooled ganache into cream whipped separately. If you fold it in while it's still warm, it'll melt the air pockets in the whipped cream and you'll get a runny mousse that won't hold between sponge layers. Milk chocolate specifically because it's softer-set and has enough sugar already that this layer doesn't need extra — any milk chocolate above about 30% cocoa solids will set properly; cheaper ones with more vegetable fat than cocoa butter can stay greasy and never firm up.

The dark ganache (120g dark chocolate, 70ml cream, 35g sugar) is more concentrated — less cream relative to chocolate — because it doesn't need whipping into anything, it needs to hold its shape coming out of a piping bag and sit glossy on top. The added sugar here is doing a different job than in the mousse: it's balancing the higher cocoa percentage of straight dark chocolate so the top decoration doesn't taste sharply bitter against the sweeter layers underneath.

The milk brushed between layers isn't decoration, it's moisture insurance. A sponge this light, cut into three thin layers, dries out fast at the cut surfaces — brushing with milk (any kind, whatever's in the fridge for your tea works fine) stops the exposed crumb from drinking moisture out of the mousse instead, which is what makes a cake taste dry by day two even though the sponge itself was fine on day one.

Cocoa biscuits pressed onto the sides, held up by strips of baking paper slid underneath, solve a real domestic problem: in a kazan-less, standard UK kitchen you've no cake-decorating turntable and no steady hand for perfectly smooth sides. The biscuits hide any unevenness in the mousse coating and give you something to hold onto structurally while it sets in the fridge. Any dry, crisp biscuit with a cocoa or plain flavour that won't go soggy fast will do — digestives work if that's what's in the cupboard, though they'll read sweeter than a proper cocoa biscuit.

Keep it refrigerated until serving, and that's not caution for its own sake — the mousse is whipped cream at heart, and whipped cream softens and slumps at room temperature within an hour or two. This is a cake that holds its three-layer shape because it's cold, not because it's structurally bulletproof. Pull it out shortly before cutting, not the moment guests arrive.