Start by melting the butter and cooling it down. Heat the oven to 180C/fan 160C/gas 4. Beat the eggs with the sugar in a mixer until pale and fluffy, then fold in the flour, vanilla pudding, baking powder, vanilla and cooled butter to make a smooth batter.

Pour 3/4 of the prepared dough into a 950g loaf tin or 20cm round lined cake tin, and add the cocoa to the remaining batter. Pour the chocolate mixture down the centre of the pale batter. Bake for 40 mins. After baking, cool the cake and sprinkle it with icing sugar or cover it with chocolate glaze, if you prefer.