1. Preheat the oven to 200C . Lightly grease a 25×11×7.5 cm loaf tin and line the base with baking paper.

2. If using the vanilla bean, split it down the middle and scrape the seeds into a bowl. Alternatively, put the vanilla extract in a bowl.

3. Add the butter and sugar, then cream using electric beaters until pale and fluffy. Add the eggs one at a time, beating well after each addition.

4. Sift the flour into a bowl. Using a large metal spoon, gently fold the flour into the butter mixture, alternating with the milk, until just combined and almost smooth. Spoon half the batter into another bowl.

5. Combine the cocoa powder and extra milk in a small bowl and stir until smooth, then stir into one of the bowls of cake batter, mixing well.

6. Spoon the two cake batters into the prepared tin in alternating spoonfuls. Using a metal skewer, 'cut' through the mixture four times to create a marbled effect.

7. Bake for 50–60 minutes, or until a skewer inserted into the centre of the cake comes out clean. Remove from the oven and leave to cool in the tin for five minutes, before turning out onto a wire rack to cool completely.

Marble cake will keep for 3 to 4 days, stored in a cool place in an airtight container, or can be frozen in an airtight container for up to 6 weeks.