Put the butter, Biscoff and marshmallows in a large bowl and melt in 30 second blasts in the microwave, stirring between each burst until fully melted.
Stir in the rice krispies until well combined.
Press into the lined tray and pack down well (if your hands keep sticking, try rubbing some butter on them).
Melt the white chocolate gently in the microwave in 30 second bursts, stirring between each burst.
Pour the chocolate on top of the rice krispies and gently spread to the edges.
Top with the Biscoff Cookies, leaving some whole and breaking others into pieces, or crushing into crumbs.
Place in the fridge for 30 minutes to set, then slice and enjoy.