Melt the chocolate and butter over a bain - marie, which is really just a fancy way of saying
dump the globs of black and yellow into a heatproof bowl and melt over simmering (not boiling) water.
Instead of dealing with
globs of chocolate stuck to the inside of a bag, I melt the peanut butter and chocolate in a large microwave - safe bowl with a cover, mix in the cereal,
dump in a few generous scoops of powdered sugar, tightly cover the bowl, and then shake.