Authentic Italian Espresso Tiramisu with Silky Mascarpone Sabayon

Tiramisu has no gelatin, no starch and no baking, so the thing holding it together is a sabayon. Whisking egg yolks with sugar over gently simmering water does two jobs at once. The sugar dissolves and the yolk proteins begin to unfold, and because you are beating air into the mixture it becomes a pale, thick foam that roughly triples in volume. Sugar also raises the temperature at which yolk proteins set, which is why you can take it to 71°C (160°F) without it scrambling. That number matters for more than texture: held there for fifteen or twenty seconds, the yolks are pasteurised. Stop whisking and the foam collapses. Stop watching the heat and the eggs catch. You are looking for a mixture that has gone from dark yellow to pale butter, and which falls from the whisk in a thick ribbon that sits on the surface for a second before it sinks.

Mascarpone is not cream cheese with a different label, and it cannot be swapped for one. Both start with cream and acid, but mascarpone is set with a little citric or tartaric acid and drained briefly, leaving it at 45 to 50% milkfat with no gums, no stabilisers and almost no acidity of its own. Cream cheese is cultured, salted and stabilised with locust bean gum, sits nearer 33% fat, and tastes tangy. Fold it into a sabayon and you get something denser, sharper and faintly grainy, and the stabilisers mean it never quite melts into the foam. Ricotta is worse still: a grainy, low-fat curd carrying far too much water. If mascarpone is genuinely unavailable, full-fat cream cheese loosened with an equal volume of thick cream is the least bad substitute, and it still will not taste like the real thing.

Savoiardi are the right biscuit because they are almost entirely air. They are piped sponge, baked until dry, with an open crumb held up by a network of tiny pores. Dip one in coffee and liquid is drawn into those pores by capillary action, fast. That is why the timing is so tight. A second per side wets the outer millimetre or two and leaves the centre dry and firm. Leave it in for five seconds and the coffee reaches the core, the crumb walls soften, and the biscuit loses the structure holding the cream up. Soft bakery ladyfingers, already moist, skip straight to that collapse. The dry core also keeps drinking after assembly, so what went in crisp comes out as cake. That is the trick of the whole dessert: you are not soaking the biscuits, you are starting a process they will finish on their own in the fridge.

None of the usual shortcut ingredients belong in this. Gelatin would set the cream into a wobbly jelly that tastes of nothing, and it is not needed: the set comes from mascarpone butterfat firming up at fridge temperature and from the sabayon's egg proteins, which is why the texture stays soft and spoonable rather than rubbery. Baking is out for the same reason. The biscuits are already cooked and the cream is a raw-set custard, so heat would set the eggs into a firm slab and drive the coffee back out of the biscuits. Cream is the more interesting case: folding in whipped cream makes a lighter, more mousse-like dessert, but it dilutes the fat and protein that give the layers their slice, so you trade clean edges for air. The denser classic version is the better one here.

The coffee does more work here than most people give it credit for. Use espresso or very strong moka coffee, not a mug of filter. Filter coffee is mostly water carrying a modest amount of flavour, so a one-second dip delivers too much liquid and too little coffee, and the biscuit goes soft before it tastes of anything. Espresso is concentrated, so a fast dip transfers real flavour without flooding the crumb. It has to be cool, not warm: hot coffee softens the biscuit faster, and warm biscuits under the cream lift the mascarpone above the temperature at which its fat is stable. It also has to be unsweetened, because there is already 100g of sugar in the sabayon and sugared coffee pushes the whole thing towards cloying. What you want from the coffee is bitterness, and sweetening it takes away the only thing on the plate that pushes back against the mascarpone.

Three separate things are happening in the fridge, and none of them are fast. First, the biscuits are still drinking. They go in crisp and finish cake-like, and that equalisation takes hours: cut a tiramisu at three hours and you will find dry biscuit inside a wet outside. Second, the fat is setting. Mascarpone is soft at room temperature and needs sustained time below about 5°C before the butterfat firms up enough to hold a slice. Third, the flavours are merging, as coffee, cocoa, alcohol and vanilla diffuse between the layers. Six hours is the floor at which it will hold a cut, eight is what this method asks for, and twenty-four is where it stops tasting like four ingredients in a dish. This is a dessert to make the day before you need it, not the morning of.

The traditional tiramisu is a raw-egg dessert, and it is worth being straight about what that means. The yolks here are cooked, but only just: the sabayon reaches 71°C (160°F) and is held there for fifteen to twenty seconds, which pasteurises them. Pasteurisation depends on temperature and time together, and 71°C is roughly the point at which Salmonella is reduced by a factor of a million within a few seconds. Sugar and constant whisking are what stop the yolks scrambling at that heat. The whites are not cooked at all. If you are cooking for anyone pregnant, elderly, very young or immunocompromised, use pasteurised carton whites with a pinch of cream of tartar, or swap them for whipped cream. Because the yolks are cooked, this is a safer dessert than a raw-egg mousse, but it is still a cold one: hold it below 4°C and treat three days as the limit.