Lightly crush or chop the Biscoff biscuits into a large bowl and add in the chopped chocolate and mini marshmallows. (You can use regular sized marshmallows and chop them in half or quarters.)
Melt the butter, white chocolate and biscoff spread together and pour through the the rest of the ingredients.
Mix quickly and press into the prepared tin in an even layer.
Press in the 65g of Biscoff biscuits into the top for decoration and place in the fridge for one or two hours until set.
Once set, drizzle over the remaining Biscoff spread and cut with a sharp knife into pieces and serve.
Store in an airtight container in the fridge for a week. I even kept mine for two weeks in the fridge and it was fine.
Video
Notes
* If you can't get your hands on Biscoff biscuits, any spiced biscuit will do. Or you can use Graham crackers or digestives instead.