Step 1: Prepare the Baking Tin

1. Line an 8×8 inch square baking tin with parchment paper, ensuring some overhang for easy removal.

Step 2: Crush the Biscuits

1. Place the digestive biscuits in a large plastic bag or a mixing bowl.
2. Crush them using a rolling pin until you have fine crumbs with some larger pieces for texture.

Step 3: Melt the Butter and Chocolate

1. In a saucepan, melt 150g (5 oz) unsalted butter over low heat.
2. Add 3 tbsp golden syrup and stir until fully combined.
3. Break 200g (7 oz) dark chocolate into small pieces and add to the saucepan along with 2 tbsp cocoa powder.
4. Stir continuously until the chocolate melts and the mixture is smooth and glossy. Remove from heat.

Step 4: Combine Dry Ingredients

1. In a large mixing bowl, combine the crushed digestive biscuits, 75g (⅓ cup) dried cranberries or raisins, and 75g (⅓ cup) chopped nuts (if using).
2. Pour the melted chocolate mixture over the dry ingredients and stir well to coat everything evenly.

Step 5: Assemble the Tiffin Base

1. Transfer the biscuit mixture to the prepared baking tin.
2. Press the mixture firmly into the tin using the back of a spoon or spatula, ensuring an even, compact layer.

Step 6: Prepare the Chocolate Topping

1. In a microwave-safe bowl or double boiler, melt 200g (7 oz) milk or dark chocolate with 1 tbsp unsalted butter, stirring until smooth and fully melted.
2. Pour the melted chocolate over the biscuit base and spread evenly using a spatula.

Step 7: Chill the Tiffin

1. Place the tiffin in the refrigerator for at least 2 hours to set fully.
2. Once set, remove it from the fridge and let it sit at room temperature for 5 minutes before slicing it into 16 squares.

Step 8: Slice and Serve

1. Use the parchment overhang to lift the tiffin from the tin.
2. Slice into 16 pieces and serve.