Step 1: Preheat the oven

Preheat your oven to 180°C (350°F). Grease and line a 20cm (8-inch) cake tin with parchment paper.

Step 2: Prepare the batter

In a large mixing bowl, beat the softened butter and caster sugar together until light and fluffy. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.

Step 3: Combine dry ingredients

Sift the self-raising flour and baking powder into the mixture. Add the milk and gently fold everything together until you have a smooth batter.

Step 4: Separate the batter

Divide the batter evenly into two bowls. In one bowl, mix the cocoa powder with the hot water to form a paste, then fold it into the batter until fully combined. Leave the other bowl as vanilla batter.

Step 5: Layer and swirl

Spoon alternating dollops of the vanilla and chocolate batters into the prepared cake tin. Use a toothpick or skewer to gently swirl the batters together to create a marbled effect. Be careful not to overmix.

Step 6: Bake

Place the tin in the preheated oven and bake for 50-60 minutes, or until a skewer inserted into the center comes out clean.

Step 7: Cool and serve

Allow the cake to cool in the tin for 10 minutes before transferring it to a wire rack to cool completely. Slice and enjoy!