Blueberry Cake

Butter, room temperature, not melted — you need it soft enough to trap air when beaten with the sugar, which is the only leavening this crumb gets besides the baking powder. Melted butter can't hold a bubble; it just coats the sugar crystals and sits there. Room temperature for an English kitchen in a centrally heated flat usually just means taking it out an hour before you start — don't microwave it, that's the melted-butter mistake by a different route.

Caster sugar rather than granulated because the finer crystal dissolves into the fat faster during creaming — less grinding time, less risk of overbeating the butter into a greasy mess trying to force coarse sugar to dissolve. Vanilla sugar is a small flavour top-up; if you don't keep a jar of it, a few drops of vanilla extract added with the eggs does the same job.

Eggs go in one at a time, and the reason isn't fussiness — it's that butter and egg (mostly water) don't naturally want to mix, and a creamed-butter emulsion can curdle, go grainy and separate if you dump four eggs in at once. One at a time, beaten in properly, keeps the fat holding onto the water instead of weeping it out. If it does split, a spoonful of the flour beaten in brings it back — that's not a secret, that's just starch soaking up the loose liquid.

Flour, baking powder, salt and lemon zest get mixed dry before they ever touch the batter, so the baking powder is distributed evenly rather than landing in one pocket that puffs while the rest stays flat. Folding, not beating, once this goes in — you've built air into the butter-sugar-egg mixture and vigorous stirring now just knocks it back out, which is the real reason "overmixed" cakes come out dense and tight rather than any mystique about gluten alone, though gluten development from over-stirring plain flour doesn't help either.

Frozen blueberries for the inside of the cake, specifically — their skins are firmer straight from the freezer and they don't collapse and weep the second they meet batter at room temperature, which is what fresh berries do, turning a clean crumb grey-purple around each one. Some say you must use fresh fruit for a "proper" cake — no, because here the frozen fruit is doing a mechanical job, holding its shape through baking, and fresh would actively work against you.

For the frosting: butter again at room temperature for the same creaming reason, icing sugar because its fineness dissolves smooth into fat without the grittiness granulated sugar would leave — you're not baking this part, there's no heat to dissolve coarse sugar for you. Cream cheese goes in last and briefly, just enough to loosen and flavour the buttercream without melting it into soup; cream cheese is soft and wet compared to butter, so overbeating it thins the whole frosting past the point of holding a swirl.

The blueberry juice for the frosting is made by simmering berries just to a gentle boil — not a rolling boil, which would drive off the fresh fruit flavour and reduce it to a jammy single note — then straining through a sieve to get a clean, seedless-skin liquid you can fold into the buttercream without lumps breaking the texture.

This feeds eight, cut into slices the width of two fingers rather than wedges, since it's baked in a loaf tin — nearer to a teatime cake than a dinner-party centrepiece, which fits well enough with having it alongside a cup of tea with milk rather than needing a fork-and-plate occasion. Patience here is mostly about the cooling stage: the cake must come fully to room temperature on a wire rack before frosting goes anywhere near it, and that's not a step you can rush by putting it in a cold larder — a warm centre inside a cool-looking crust will still melt butter frosting from underneath.