Marbled pound cake

Heat 3 tbsp neutral oil in the microwave about 30 seconds, hot but not boiling.

Stir 3 tbsp cocoa powder into the hot oil until smooth and leave it to bloom. Blooming cocoa in hot water or fat makes it taste more intense, which matters when using as little as possible for texture.

Weigh an empty small bowl and note the weight, to subtract later.

Soften 340 g butter, in the microwave a few seconds at a time, rotating between bursts, stopping as soon as it smooshes. Parts still firm will even out.

Add 400 g granulated sugar to the butter and beat several minutes until it looks whipped and goes slightly pale. The sugar acts as an abrasive that cuts micro-bubbles into the butter, and those expand in the oven; skip this and the cake is a brick.

Beat in 237 ml cold cultured buttermilk and whip again until noticeably fluffier.

Add four eggs one or two at a time. All at once gives a curdled batter that the beaters cannot grab, and the extra mixing needed can deflate the foam and toughen the cake.

Mix in a glug of vanilla, 1/4 tsp baking soda and a tiny pinch of salt.

Work in 420 g all-purpose flour one cup at a time, just until smooth; do not overmix.

Grease a 9x5 inch loaf pan and dust every surface with flour.

Weigh the batter bowl, subtract the bowl weight and halve the result. Here half is 800 g, so 800 g goes into the pan, kept to one side. Halving by weight rather than eye keeps the chocolate batter from being too thin.

Beat the bloomed cocoa and oil into the remaining batter, scraping down and folding to a homogenous colour. It will look wetter than the plain batter, which the chocolate needs. Cocoa without the oil bakes up dry; with it the texture is like using melted solid chocolate. A teaspoon or two of instant espresso powder may go in here.

Drop the chocolate batter into the other half of the pan, and either bake as is or swirl a little with a knife. Too much swirling mixes the two flavours and defeats the point.

Bake at 350F, 180C. With a convection fan, adjust down per the manufacturer: 325F, 160C here, which makes the oven effectively hotter and improves heat transfer.

A loaf this thick and dense takes nearly an hour and a half. Too hot an oven burns the outside before the middle is done.

Test with a toothpick right under the apex, the last part to bake. Clean toothpick means done.

Cool and solidify about half an hour before turning it out; run a knife round the sides and give it a thump.

Slice with a serrated bread knife, carefully through the crunchy top crust, which tears or crumbles when fresh.