Preheat the oven to 160℃. Grease and line a 1 kg loaf tin with baking paper.

In a large mixing bowl using an electric whisk, beat the softened butter and the caster sugar until creamy.

Add half of the eggs and beat them in, then add the remaining eggs and beat until well combined.

Pour in the milk, vanilla essence and add the self-raising flour. Fold it through until the mixture is well combined and you have a smooth cake batter.

Divide the mixture equally between two mixing bowls.

Add the cocoa powder into one of the bowls and fold it through until well combined. If it looks too dry then add a splash of milk.

Dollop a spoon of one of the mixtures into the loaf tin, followed by the other. Keep alternating until all the batter is used up.

Give the tin a shake and a bang on the kitchen surface so that it spreads into the edges.

Bake it for 55 minutes or until a skewer comes out clean.

Leave it to cool in the tin for a few minutes before transferring it to a cooling rack to cool completely.