01 - Line a 9x9-inch square pan with parchment paper and lightly grease the surface to prevent sticking.
02 - In a large saucepan over low heat, melt the unsalted butter. Add Biscoff spread and stir continuously until the mixture is smooth and fully blended.
03 - Add mini marshmallows to the saucepan. Cook over low heat, stirring constantly, until the marshmallows are completely melted and incorporated.
04 - Remove the saucepan from heat. Gently fold in the Rice Krispies cereal until each piece is thoroughly coated with the marshmallow-Biscoff mixture.
05 - Transfer the mixture into the prepared pan. Using a lightly greased spatula or wax paper, press the mixture evenly and gently into the pan to form an even layer.
06 - Drizzle the melted Biscoff spread over the top and sprinkle with crushed Biscoff cookies if desired.
07 - Allow to cool at room temperature for about 45 minutes, or until completely set. Cut into 16 equal squares before serving.