1. Line an 8x10-inch tin with baking paper. Let the paper hang over the edges to make it easier to remove the tiffin after refrigerating.

2. Add the Digestive and Biscoff biscuits to a large bowl. Use the end of a rolling pin to crush them into rough chunks. Make sure the pieces are not too small; it's best to have a few crumbs with larger pieces throughout.

3. Add the margarine, golden syrup, cocoa powder and the semi-sweet chocolate to a pot. Turn the heat to medium-low and stir occasionally until everything has melted and there are no lumps left. Remove the mixture from the heat and let it cool for a few minutes.

4. Pour the chocolate mixture over the crushed biscuits and add the white chocolate chips if using. Stir until everything is evenly coated in the chocolate.

5. Transfer the mixture to your lined tin and press it tightly into place (I used the bottom of a cup to do this). This helps to prevent the tiffin from breaking when you cut it.

6. Place the tiffin in the fridge while you make the topping.

For the topping:

1. Melt the white chocolate and the Biscoff spread until runny. Spoon it over the tiffin then tilt the tray to move the chocolate and Biscoff spread around to create a marbled effect.

2. Once the top is evenly covered, sprinkle over the Biscoff biscuit crumbs and place in the fridge for 3 hours.

3. When the tiffin has set, remove it from the tin using the baking paper.

4. Run a sharp knife under boiling water and wipe it dry with a dishcloth. This will prevent the chocolate topping from cracking.

5. Carefully cut the tiffin into 12 square slices and enjoy!