Preparation
Preheat the oven to 180°C (160°C fan, gas 4). Grease a ring cake tin with butter and sprinkle with crumbs.
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Whisk the butter until soft and creamy.
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Whisk in the sugar until creamy.
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Add the vanilla sugar and salt.
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Grate the zest from the lemon and beat into the batter.
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Gradually add the eggs, whisking constantly.
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Add some milk.
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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.
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Spoon half the mixture into the prepared tin.
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Mix the cocoa powder into the other half. Add a little milk if necessary.
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Spread the chocolate mix evenly over the plain mix and pull a fork randomly to swirl the mixtures together.
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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.
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