Marble butter cake

Preheat oven to 180ºC. Line a large loaf tin with baking paper. Put butter, sugar, zest and cornflour in the bowl of an electric mixer and beat on high for 3 minutes or until light and creamy. Add eggs and vanilla. Beat until smooth.

Sift the flour and baking powder into the bowl and mix until smooth. Put one-third of the mixture in a medium bowl, then fold in chocolate.

Spoon vanilla mixture into prepared tin, then add the chocolate mixture in parallel lines. Stir gently with a teaspoon to marble. Do not overmix. Bake for 15 minutes or until the skin is just set. Use a sharp knife to make a shallow slash along the top of the cake, then bake for a further 30 minutes or until a skewer inserted in the centre comes out clean. Cool completely on a wire rack. Serve.