Citrus Cake

Butter has to be properly soft, not melted — melted butter can't hold air bubbles, it just coats the sugar and sits there. Some say warm it a little in the microwave to speed things along: no, because a half-melted edge collapses the structure before you've even started, and the whole cake rides on that structure. Leave it out a few hours; if your kitchen's cold, a warm radiator shelf does the job, not heat applied directly.

Icing sugar plus caster sugar plus vanilla sugar in the main batter isn't fussiness — icing sugar dissolves instantly and gives a finer, tighter crumb, caster sugar holds a bit more structure and sweetness, vanilla sugar is there for scent rather than sweetness at all. If you've only got caster sugar in the cupboard, use it for the lot; the cake will be very slightly coarser and you won't notice in a loaf this rich.

Eggs and yolks go in separately, one at a time, for the same reason the butter has to be soft: each one has to be beaten fully into the fat before the next goes in, or the batter splits — you'll see it curdle, fat and liquid refusing to hold together — and a split batter traps less air and bakes heavier. If it does split, a spoonful of your measured flour beaten in will pull it back together; that's not cheating, that's just giving the egg something to grip onto.

Flour and baking powder get sifted together and folded in last and briefly, because flour develops gluten the moment it's wet and worked, and gluten in a cake is what you don't want — it's what makes bread chewy and cake tough. Mix until the flour disappears and stop. Plain flour, not self-raising, because you're controlling the lift yourself with a measured amount of baking powder; self-raising flour's built-in rise varies by brand and you lose that control.

Marzipan is almond paste bound with sugar — in Britain it's cheap and reliable off the supermarket shelf, which is itself a small piece of economics: almonds were once the luxury, now the sugar-bound paste is an everyday baking aisle item, so a cake that reads as special costs you very little. It has to be cut small before mashing with sugar, because a lump of marzipan won't break down in a mixer the way butter does — it's dense and slightly oily rather than creamy, and left in chunks it stays as chunks in the finished cake. If you can't get marzipan, ground almonds plus a little almond extract and extra sugar gets you most of the way, though you lose the dense, slightly chewy pocket marzipan gives.

The lemon zest for the icing gets dried on a paper towel for the hour the cake bakes and cools — not for show, but because wet zest sinks and bleeds colour into white icing, dry zest sits on top and stays where you put it. A plate over the cooling cake instead of a proper cake dome works fine in a normal kitchen; you just want the zest somewhere open to the air, not sealed under anything.

Tin size matters more than most recipes let on: 24 cm is wide enough that the centre of this heavy, almond-dense batter actually cooks through in the time given rather than staying wet in the middle while the edges dry out. Go smaller and you're either under-baked in the centre or burnt at the edge — there's no good version of that trade. "Fully cooked through" is a skewer coming out clean, not a clock; forty-five minutes is where to start checking, not where to stop.

This feeds twelve in reasonably restrained slices, which given the butter content is probably wise — it's a cake for a crowd with tea rather than a quick slice standing at the counter. Patience is the real ingredient twice over here: once at the start, letting butter and sugar actually go pale and fluffy rather than just combined, and once at the end, letting the cake cool properly before icing goes anywhere near it — warm cake melts icing straight off the top and you lose the finish for nothing.