Step 1: Preheat the Oven and Prepare the Tin

• Preheat your oven to 180°C (350°F) or 160°C (fan).
• Grease and line a 9-inch round or loaf cake tin with parchment paper.

Step 2: Cream the Butter and Sugar

• In a large mixing bowl, cream the softened butter and caster sugar together until light and fluffy. This will take about 2-3 minutes with an electric mixer.

Step 3: Add the Eggs and Vanilla

• Add the eggs one at a time, mixing well after each addition.
• Stir in the vanilla extract for a smooth, fragrant batter.

Step 4: Add the Dry Ingredients

• Sift in the self-raising flour and baking powder. Gently fold into the mixture until combined.
• Add the milk to achieve a smooth batter consistency.

Step 5: Divide and Flavor the Batter

• Divide the batter evenly into two bowls. In one bowl, fold in the cocoa powder until fully incorporated to create the chocolate batter.

Step 6: Create the Marble Effect

• Spoon alternating dollops of vanilla and chocolate batter into the prepared cake tin.
• Using a skewer or knife, gently swirl through the batter to create a marbled effect. Be careful not to overmix, as this will blend the colors rather than create distinct swirls.

Step 7: Bake the Cake

• Bake in the preheated oven for 45-50 minutes or until a skewer inserted into the center comes out clean.
• Allow the cake to cool in the tin for 10 minutes, then transfer to a wire rack to cool completely.

Step 8: Add Topping (Optional)

• Once cooled, dust the cake with icing sugar or drizzle with a chocolate glaze for a finishing touch.