Preheat the oven to 180°C (160°C fan, gas 4). Grease a ring cake tin with butter and sprinkle with crumbs.
Whisk the butter until soft and creamy.
Whisk in the sugar until creamy.
Add the vanilla sugar and salt.
Grate the zest from the lemon and beat into the batter.
Gradually add the eggs, whisking constantly.
Add some milk.
Sieve the flour with the baking powder and starch and add to the batter, alternating with the milk until a soft dropping consistency is formed.
Spoon half the mixture into the prepared tin.
Mix the cocoa powder into the other half. Add a little milk if necessary.
Spread the chocolate mix evenly over the plain mix and pull a fork randomly to swirl the mixtures together.
Bake in the preheated oven for about 1 hour. Test with a skewer. Remove from the oven, allow to cool slightly, turn out of the tin and cool completely. Sprinkle with icing sugar and serve in slices.