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Heat the oven to 180C/160C fan/gas 4. Butter and line the bases of two 18cm sandwich tins. Beat the butter, sugar, flour and eggs together in a large bowl using an electric whisk until it's lump-free.
Divide the mixture between two bowls. Beat half the jam and the food colouring, if using, into one, and beat the vanilla into the other.
Spoon alternating dollops of the mixes into the prepared cake tins, then swirl together using a skewer. Do this carefully – if you overdo it, you won’t see the pattern. Smooth the tops with the back of a spoon.
Bake for 20-25 mins until golden and a skewer inserted into the centre comes out clean. Leave to cool in the tins for 10 mins, then turn out.
Meanwhile, to make the white chocolate layer, put the chocolate and cream in a heatproof bowl set over a pan of just simmering water, ensuring the bowl doesn’t touch the water. Stir until the chocolate has melted into the cream and you’re left with a smooth, glossy mixture. Leave to cool completely in the fridge, stirring occasionally.
Sandwich the cakes together with the remaining raspberry jam and the white chocolate mixture, then generously dust the top with icing sugar. Slice and serve.