Melt your white chocolate. This part is tricky because white chocolate can be finicky. Your best bet is probably to do it slowly on the stove. You can also microwave it, just be sure not to cook too long without stirring. Heat and mix till melted. Also warm the marshmallow fluff slightly.
Put your Chex in a big bowl and then pour your white chocolate mixture over it and stir till it’s covering the Chex evenly. Then add the warm marshmallow fluff and stir it in. It’s likely to be a little bit clumpy. It will still taste GREAT!
Sprinkle on your powdered sugar and mix till it covers the Chex.
Sprinkle on mini marshmallows and Sixlets and sprinkles.