In a medium bowl, combine 1 cup of white chocolate chips and butter. Microwave in 20 second increments until it is melted.
Add peanut butter and marshmallow fluff. Microwave for another 20 - 30 seconds. Stir well so everything is combined (it will be pretty thick).
Pour Chex Cereal into a large bowl. Gently stir in peanut butter fluff mixture. Keep stirring until mixture coats the cereal. It will take a little while - so keep stirring.
Use your hands to break apart any large clumps of cereal.
Add powdered sugar and stir until cereal is coated.
Add sprinkles and remaining 1/2 cup of chocolate chips.