01 - Line a 9x9-inch square baking pan with parchment paper and lightly grease with nonstick spray or butter.
02 - Melt unsalted butter in a large saucepan over low heat. Stir in Biscoff spread until smooth and fully combined.
03 - Add mini marshmallows to the saucepan and cook, stirring constantly, until completely melted and mixture is smooth.
04 - Remove saucepan from heat. Add Rice Krispies cereal and fold gently using a rubber spatula until evenly coated.
05 - Using a greased spatula or wax paper, press mixture evenly into the prepared pan without compacting too tightly.
06 - Drizzle melted Biscoff spread over the surface and sprinkle crushed Biscoff cookies for added crunch, if desired.
07 - Allow to cool at room temperature for about 45 minutes or until firm and set.
08 - Cut into 16 equal squares and serve as a sweet treat.