In a large pot, melt butter and marshmallows together until smooth. Take off heat and stir in biscoff spread. Pour in cereal and stir until cereal is sticky and every grain is coated. You may have to stir a little more than when making plain rice krispie treats. Pour into a greased 8x8 dish, press down to even the top and cool to room temperature. Cut into 9 even squares and serve.