1 hr 10 min.
Pre-heat the oven to 180°C | 350F | gas 4. Grease and line a 2lb loaf tin with greaseproof paper.
Whisk together all the ingredients apart from the cocoa powder in a large mixing bowl for 2-3 minutes using an electric hand-held whisk until smooth.
Divide the mixture into two separate bowls. Whisk the cocoa powder into one mixture to create the chocolate batter.
Spoon half of the vanilla batter into the loaf tin. Carefully spoon 2/3rds of the chocolate batter carefully on top.
Spoon the remaining vanilla batter on top then top that with the remaining chocolate batter on top.
Bake for 50-60 minutes until a cake tester comes out clean when inserted into the centre.
Remove when ready and allow to cool in the tin for 5 minutes before turning out onto a wire rack.
Remove the greaseproof paper and cut into slices before serving on a plate.