Start by melting the white chocolate and butter in a heatproof bowl over a pan of simmering water. Be careful not to let the water touch the bowl otherwise the chocolate can burn.
Take it off the heat, then beat in the Biscoff spread until smooth.
Add the crushed biscuits and mini marshmallows to the melted chocolate/Biscoff mix and stir until it’s well combined.
Pat it down into a lined 20cm tin, then drizzle over some melted Biscoff spread and sprinkle on some more crushed biscuits.
Pop it into the fridge for a few hours to set.
Cut it into portions and enjoy.