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Heat oven to 170C/150C fan/gas 3. Line a cupcake tin with cases. Cream the sugar with the butter using an electric whisk, until combined. Crack in the egg and add the vanilla extract, then whisk to combine. Add half the flour, the baking powder and a pinch of salt, whisk to combine, then add half the milk and whisk briefly again. Repeat to use the remaining flour and milk. Don't worry if the mixture seems loose.
Spoon about half the mixture into a separate bowl, add the cocoa and chocolate chips and stir well to combine.
Using a teaspoon, divide the chocolate mixture between the 12 cupcake cases. Use a wooden skewer to spread the mixture evenly to the edges. Rinse and dry the teaspoon then repeat with the vanilla layer, taking care not to disturb the chocolate below and spreading to the edges again.
Bake for 25 mins. Leave to cool slightly in the tin before turning out onto a wire rack to cool completely.
To make the frosting, sift the icing sugar, then combine with the butter using an electric whisk. It will look crumbly initially – persevere and it will come together. If you're icing is too stiff, add 1 tbsp milk to the mixture. Carefully add 1-2 drops of the food colouring and combine to make a pale pink frosting to ice the cakes. When you cut them in half, the coloured layers will be revealed.