Cream the butter, sugar and vanilla

• In a bowl, whisk the butter, most of the sugar and the vanilla until light and fluffy.

Whisk in the eggs

• Whisk in the eggs on a low speed, one at a time, to avoid curdling.
• It may look slightly curdled, which is fine.

Fold in the dry ingredients and milk

• Add the flour, baking powder and milk.
• Fold through until fully incorporated.

Fill the loaf tin

• Spoon two thirds of the batter into a lined loaf tin.

Make the chocolate mixture

• Add the cocoa powder, remaining sugar and remaining milk to the batter left in the bowl.
• Fold through until fully incorporated.

Layer and marble

• Spoon the chocolate mixture on top of the batter in the tin.
• Gently swirl with a fork to create a marbled effect without overmixing.

Bake the cake

• Bake at 180 °C (160 °C fan).
• Check after 50 minutes with a skewer inserted all the way through the centre; if it comes out clean the cake is ready, otherwise continue baking.

Cool the cake

• Remove from the oven and, once cool enough to handle, take the cake out of the tin.
• Let it cool on a cooling rack.

Dust and serve

• Generously dust all over with icing sugar.
• Slice into thick chunks and serve warm with full fat yogurt or crème fraîche.