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Heat the oven to 200C/180C fan/gas 6 and line a 20cm square tin with baking parchment, leaving some overhanging. Make the shortbread by beating 170g of the butter and the sugar using an electric whisk until fluffy. Sift over the flour and cornflour and stir to make a dough. Press into the tin, making sure it’s level. Prick all over using a fork and bake for 15-20 mins until set and lightly golden. Leave to cool completely in the tin.
Meanwhile, make the caramel. Put the butter, condensed milk, golden syrup and dark brown sugar in a medium saucepan with a pinch of salt over a medium-low heat, and cook for 10-12 mins, stirring constantly, until thick, darkened and fudgy. Add the rum and stir to combine – it will bubble up so be careful. Remove from the heat, stir in the raisins and leave to cool for 5 mins, before pouring onto the cooled shortbread base. Leave to cool and set completely.
Melt the dark chocolate and the remaining 50g butter together in short bursts in the microwave until glossy, or in a heatproof bowl set over a pan of simmering water, ensuring the bowl doesn’t touch the water. Melt the milk chocolate in a separate bowl. Pour the dark chocolate over the set caramel layer then drizzle over the milk chocolate and swirl with a skewer. Put in the fridge to set. Slice into 16 squares and put in a gift box lined with baking parchment. Will keep in an airtight container for three days.