Step 1: Preheat the oven

Preheat your oven to 180°C (160°C fan). Grease and line an 8-inch round cake tin with baking paper.

Step 2: Cream the butter and sugar

In a large mixing bowl, beat the softened butter and caster sugar together using an electric mixer until light and fluffy, about 3-5 minutes.

Step 3: Add the eggs

Add the eggs one at a time, beating well after each addition. If the mixture begins to curdle, add a spoonful of flour to bring it together.

Step 4: Mix in the flour and milk

Sift the self-raising flour into the bowl and fold it in gently using a spatula. Add the milk and vanilla extract, mixing until smooth and well combined.

Step 5: Divide the batter

Divide the batter evenly between two bowls. In one bowl, add the cocoa powder and mix until fully incorporated, creating the chocolate batter.

Step 6: Layer and swirl the batters

Spoon alternating dollops of vanilla and chocolate batter into the prepared cake tin. Use a skewer or knife to gently swirl the batters together to create a marbled effect. Be careful not to overmix, as this will lose the marbling pattern.

Step 7: Bake the cake

Bake in the preheated oven for 45-50 minutes, or until a skewer inserted into the center of the cake comes out clean. The top should be golden and the cake should spring back when lightly touched.

Step 8: Cool and serve

Allow the cake to cool in the tin for 10 minutes, then transfer it to a wire rack to cool completely. Dust with powdered sugar or serve as is.