Butter and line a 15–20cm baking tin.
In a saucepan: melt the butter, sugar, golden syrup, and cocoa powder. Once combined, stir in raisins and crushed biscuits.
Pour the mixture into the prepared tin and press down so it's all level.
Break up the chocolate into a microwaveable bowl and place in the microwave for 10 second intervals, stirring in between, until completely melted.
Pour the liquid chocolate over the mixture in the tin until evenly coated.
Cool in fridge for minimum of two hours. Slice and serve!