Line a 20cm (8-inch) square baking tin with parchment paper, leaving an overhang on two sides for easy removal. Set aside.

Place the 400g of Biscoff biscuits in a sealed bag and crush with a rolling pin to a mix of fine crumbs and small chunks. Transfer to a large heatproof bowl.

In a medium saucepan over low heat, melt the butter, golden syrup, and brown sugar together, stirring until smooth and the sugar has dissolved. Do not boil.

Remove the saucepan from the heat. Immediately add the 200g of white chocolate and 100g of Biscoff spread. Stir until completely melted and smooth. Stir in the vanilla and salt.

Pour the warm chocolate mixture over the crushed biscuits. Mix thoroughly until every piece is coated.

Press the mixture very firmly and evenly into the prepared tin.

For the topping, gently melt the 150g white chocolate and 50g Biscoff spread together. Pour over the base and spread evenly.

Sprinkle the crushed biscuit decoration over the wet topping. Chill in the refrigerator for at least 4 hours, or overnight, until completely set.

Lift the set cake from the tin using the parchment paper. Use a sharp knife to slice into 16 squares. Serve chilled.