Preheat oven to 180°C/350°F. Grease a deep 20cm /8-inch square cake pan or a 1 lb loaf tin. Line base and sides with baking paper or butter the tin and dust with a layer of flour.

Start with the standard cake batter method or creaming the butter and sugar. Using your stand mixer, hand mixer or wooden spoon, beat the butter, vanilla extract and sugar until light and fluffy. You will see the colour change from a yellow to a lighther yellow.

On a slower speed, beat in eggs, one at a time. Alternate the eggs with spoonfuls of the flour. Add spoonfuls of the milk as you go too. Beat for about 20 seconds after adding each. Go by eye to see that the ingredients have been incorporated before adding the next one.

Divide the cake mixture into 2 bowls, a ratio of 2:1, vanilla to chocolate. Add the sifted cocoa powder into the smaller bowl and stir to combine. This is a rough guide as you can choose how much of each flavour you want in your cake. I find that half half results in too much chocolate.

Spoon the mixture into your baking tin, alternate between the vanilla and chocolate. You can swirl the mixture with a butter knife to create a marbling effect. Continue until all the mixture has been used up.

Bake the cake for about 45 minutes in the middle shelf. Use a skewer to test if the cake is done. It should come out clean and not have any cake batter. Then stand in the baking tin for 5 minutes to cool before turning out onto a wire rack.

Dust with sifted icing sugar before serving or just serve it plain with a mug of tea.

You can also use a 22cm (9-inch) round cake pan for this recipe. This cake will keep in an airtight container for up to 3 days. It can be frozen for up to 3 months.