Chop the Maltesers in half and roughly break up the cookies (a mixture of small and medium-sized chunks works well)
Put the chocolate, Biscoff and butter into a large pan and melt over a very low heat (for microwave instructions see tips below), stirring frequently until only a few lumps of chocolate remain. Take off the heat, stir until smooth, then let cool for 5 minutes
Remove 175g of the chocolate mix from the pan and set it aside
Stir the biscuit pieces into the remaining chocolate mixture, then fold through the chocolate chips, Maltesers and ¾ of the marshmallows
Tip into a greased and lined tin and use a metal spoon to press the mix into the corners and flatten it down into an even layer
Spoon the reserved melted chocolate mixture over the top and roughly spread it out
Scatter the remaining marshmallows over the top and use your fingers to press them in
Finally, melt the remaining Biscoff spread in a small pan over a very low heat for 10-15 seconds, until it loosens, and drizzle over the top
Set aside to cool, then chill for around 3 hours until set. Remove the baking parchment and slice into bars using a large sharp knife
Store at room temperature for up to 2 weeks in an airtight container