Black Forest Cake (Chocolate Sponge, Sour Cherries and Kirsch)

Variations

• In a glass (Schwarzwälder Kirsch im Glas): the same components layered in a tumbler with the sponge broken up. Nothing has to be cut, so the cream can be softer and the syrup more generous.
• Sides left bare, as in the photograph: chocolate on top only, with the cut face showing. For a cake whose argument is its layers, that is arguably the better presentation.
• Without alcohol: more cherry syrup in place of the kirsch. It removes the whole axis the first tip below is about, and what is left is a cherry cream cake. Treat it as a different dish.
• With sweet cherries: drop the sugar to 20g and raise the lemon juice to a tablespoon. It still will not reach a Morello's acidity.
• With a bought sponge: use less syrup. Factory sponge is wetter than this one, and the same amount will collapse it.

Tips

• Of the places alcohol goes, this cake's place is not in a pan. Alcohol in cooking closes the list at four: "So there are really four moments: before cooking (to move aroma across), early (to drive it off), midway (to keep it), and at the end (usually a mistake)". And it says so itself one line earlier — "All three of those are about the pan.". Even the fourth, a marinade, ends up boiled because that liquid becomes the sauce. Here the kirsch goes onto sponge that is already baked and cooled, and nothing after that is baked or boiled. The same guide writes of alcohol added last that "the raw bite of alcohol stays too. Usually a mistake." — and here that is the purpose rather than a fault.
• So there is no one-or-two-minute simmer to fall back on. The guide names "The commonest error is a hurried splash at the finish." and gives a short heating as the cure. This cake has no such step at all, so the quantity of kirsch is the result. Past about 40ml a sheet you have a spirit rather than a cake the next day. The only dial is putting less in to begin with, not cooking it off afterwards.
• Which also fixes the order the syrup is made in. Heat it to dissolve the sugar; add the kirsch off the heat. The guide's opening sentence is why: "Ethanol sits between the two, dissolves both, and evaporates faster than water". Poured into a boiling syrup, the ethanol carrying that aroma leaves on the spot and the sweetness is all that stays.
• That the sponge has to be dry is already settled elsewhere. Genoise says "It is deliberately a dry cake.", and names as the trade for a whole-egg foam that "the price of not being fluffy is that it survives being split into layers and brushed with syrup". Tiramisu puts the principle in one line — "only something already dry takes up liquid and still holds its structure". That is why this cake is built on a cocoa genoise, and why a rich buttery sponge leaves the syrup nowhere to go.
• But it is wetted differently from tiramisu, and that is why a brush. There a biscuit is dipped into liquid and "One to two seconds puts in roughly what the biscuit can hold.". A dipped thing can be lifted back out; a sheet of sponge cannot. Syrup once poured on is not recoverable, so this side measures it and paints it on.
• Without sour cherries the cake falls over. Against 600ml of cream, 210g of sugar and 100g of chocolate, the only thing holding the balance is the acidity of a Morello (balancing flavour). Swap in sweet cherries and you are stacking sugar on sugar; raising the lemon juice then is first aid, not a substitution.
• Thicken the cherry liquid, and use it cold. Jarred syrup used as it comes runs between the layers and waterlogs the sponge (thickening a sauce). And warm filling meeting cream collapses it — exactly as tiramisu describes its own cream: "not a gel; firm because the butter is firm, and left at room temperature it simply slumps". Whipped cream is not set; it is standing on cold fat and air.
• Covering the outside in chocolate shavings follows from that, rather than being decoration. A ganache or fondant coat has to be melted and poured, and anything hot poured over cream sinks through it. So the surface here is an already-solid thing shaved thin and pressed on. Shave chocolate at 18-20C to get curls: colder and it powders, warmer and it smears on your hand.
• Dip the knife in hot water and wipe it between slices. Cream, set chocolate and cherry filling soften at three different temperatures, and a dry blade always drags one of them.