Step 1: Prepare the Biscuit Base

1. Crush the biscuits into a mix of fine crumbs and small chunks. You can place them in a ziplock bag and crush them with a rolling pin.
2. In a saucepan over medium heat, melt the butter, golden syrup, sugar, and cocoa powder. Stir continuously until smooth.
3. Remove from heat and mix in the crushed biscuits and raisins (if using). Stir well to ensure the biscuits are fully coated.

Step 2: Shape and Set

1. Line a 20 cm (8-inch) round or square pan with parchment paper.
2. Transfer the biscuit mixture into the pan, pressing it down firmly to create an even layer.

Step 3: Prepare the Chocolate Topping

1. Finely chop the chocolate and place it in a heatproof bowl.
2. In a saucepan, heat the cream until it just begins to bubble (but do not boil).
3. Pour the hot cream over the chocolate, cover, and let sit for 3-4 minutes. Stir until smooth.
4. Pour the chocolate mixture over the biscuit base and spread evenly.

Step 4: Chill and Serve

1. Refrigerate for at least 4-5 hours (or overnight) until firm.
2. Once set, slice into squares or wedges and serve.