Butter and line a 15-20cm tin with baking parchment. In a large saucepan, melt the butter, sugar, syrup and cocoa. Stir through the biscuits and raisins.

Pour the mixture into the prepared tin and press down, then smooth the top with the back of a spoon. Microwave both chocolates in short 20 second bursts, stirring frequently, until melted. Pour them over the mixture in the tin. Use a palette knife or spoon to smooth over, so it's completely coated in chocolate.

Put the tin into the fridge and leave for about 2 hrs to set, or overnight. Run a kitchen knife under the hot tap then cut into squares.