Preheat the oven to 180°C (350°F) and grease and line a 2lb (900g) loaf tin with parchment paper.

In a large mixing bowl, cream the softened butter and caster sugar together until the mixture is light, pale, and fluffy.

Add the eggs one at a time, beating well after each addition; add a tablespoon of the flour with the final egg to prevent the mixture from curdling.

Gently fold in the remaining self-raising flour using a large metal spoon or spatula until just incorporated.

Divide the batter equally into two separate bowls.

In a small cup, whisk together the cocoa powder and hot water to form a smooth paste.

Stir the cocoa paste and 1 tablespoon of milk into one bowl of batter until uniformly chocolate-colored.

Stir the vanilla extract and the remaining 1 tablespoon of milk into the second bowl of batter.

Spoon the vanilla and chocolate batters into the prepared tin in an alternating pattern.

Use a skewer or a thin knife to gently swirl through the batters twice in a figure-eight motion to create the marble effect.

Bake in the center of the oven for 45 to 55 minutes, or until a skewer inserted into the center comes out clean.

Allow the cake to cool in the tin for 10 minutes before transferring to a wire rack to cool completely.