Grease the cake tin and line with baking paper.
Fill one piping bag with the vanilla batter and another with the cocoa batter. Fill the bottom of the cake tin with one third of the vanilla batter, then add half of the cocoa batter lengthways into the middle of the vanilla mixture. Pipe in another third of the vanilla batter and put the rest of the chocolate batter in its middle. Finish off by layering the final third of the vanilla batter across the top. Use a knife dipped in melted butter to make a split in the cake’s surface to help it rise in the oven. Bake at 300°F (150°C) for approx. 60 minutes. Check it is fully baked by piercing the cake with a knife – if it comes out clean, the cake is done. Turn out onto a rack and leave it on its side for 10 minutes so it keeps its shape.