Line 8-inch or 9-inch square baking pan* with parchment paper. Allow excess paper to hang over the sides. The overhang will help with unmolding. Set aside.
In a large pot, melt butter over medium heat. Add marshmallows. Stir until marshmallows have melted. Reduce to low heat. Add Biscoff spread and stir until melted and thoroughly combined.
Remove from heat and immediately add in rice cereal and chopped Biscoff cookies. Fold to combine until cereal and cookies are completed coated in the marshmallow mixture.
Transfer cereal mixture to prepared baking pan. Press the cereal mixture into an even layer. Cool to room temperature, about 30 minutes.
Use parchment paper overhang to lift and unmold rice krispies treats from the pan. Place on a cutting board. Use a sharp knife to slice into desired serving sizes.
Optional Topping: Place Biscoff spread in a microwave safe bowl. Heat in 10-15 second intervals, stirring each time, until mixture easily drips off the spoon. Drizzle liquid over the cut rice krispies treats.