Place powdered sugar in a medium Ziploc bag.Set side. Place rice chex in a medium mixing bowl.Place chocolate in a small microwave safe bowl and cook in 10 second increments until melted.Stir until smooth.Stir in Biscoff. Mix well until smooth.Pour Biscoff chocolate mixture over rice chex and toss to coat well. Place the chex in the Ziploc bag with powdered sugar and shake until each piece is nicely coated. Transfer into a bowl and serve.