Line a baking sheet with wax paper and grease with cooking spray. In a pot over low heat, melt butter. Add marshmallows and stir until melty. Turn off heat and immediately stir in cereal. Stir until completely coated. Press mixture onto prepared pan and pat into a thin even rectangle. Spread fluff in a single thin layer over cereal. Slice cereal sushi sheet in half crosswise, then roll up halves, starting from the long side. Refrigerate until firm, 1 hour. Slice into sushi rolls.