Blueberry Cheesecake

Biscuits: digestives are the obvious choice in a British kitchen because they're already slightly salted and not too sweet, which balances a filling that's getting plenty of sugar from elsewhere. Any plain, dry biscuit that crumbles rather than bends will do the same job — a rich tea works, a ginger nut works and adds its own warmth, Graham crackers if you can find them. Avoid anything soft or filled; you need dry crumb that can soak up butter and then go hard again in the cold.

Butter: melted, not softened, because melted butter coats every crumb evenly and then re-solidifies in the fridge, gluing the base into one piece. This is the same chemistry as a flapjack base — fat as mortar.

Double cream and icing sugar: double cream whips because it has enough fat to trap air in a stable structure; anything leaner (single cream, whipping cream at a stretch) either won't hold peaks or will hold them for a much shorter time, and the mousse will sit heavier and wetter. Icing sugar rather than caster here because its fine texture and small amount of cornflour dissolve into the cream without any grittiness and help it hold its whip a little longer — useful since you're folding it, not eating it straight away.

Ricotta: it's there for weight and a faint sourness, not sweetness. It's also forgiving — if yours is a bit wet, drain it in a sieve for twenty minutes first, or the filling will be looser than you want and take longer to firm up. Mascarpone is the obvious substitute if you want something richer and don't mind losing the slight tang; cream cheese works too but bring it to room temperature first or it'll sit in lumps when you try to beat it soft.

Blueberries and caster sugar: cooking them down isn't about making jam, it's about breaking the skins and releasing juice so the blender actually gives you a smooth purée instead of a seedy, chunky one. Low heat, stirred the whole time — you're watching for the berries collapsing and the liquid around them turning from watery to syrupy, not for a clock. If you don't know the signs of a mechanism going wrong here, trust your eyes: burnt sugar smells sharp and acrid, and you'll know immediately if you've gone too far.

Gelatine: bloomed in cold water first so it swells evenly and dissolves without clumping — drop it straight into something warm and you get little rubbery threads instead of a smooth set. 15g for this volume of liquid gives a filling that's firm enough to slice but still yields under a spoon; use less and you risk a mousse that won't support its own weight once you lift a slice, use noticeably more and it turns bouncy, more jelly than cheesecake. If you only have leaf gelatine, soak the sheets in cold water until floppy, squeeze them out, and use roughly the same weight — the method is identical.

The four hours in the fridge at the end is not a suggestion, it's the actual setting time the gelatine needs to form its network through all that cream and ricotta — rush it and you get a filling that looks set on top but slumps the second you cut into it. This is the one ingredient in the whole dish that is pure patience: there's no shortcut, no higher setting on the fridge that speeds it up, you just have to leave it alone.