Line a 13 x 9-inch baking pan with parchment paper, leaving an overhang on two sides.
Add to a large mixing bowl: cereal, 1 cup marshmallows, 1 cup butterscotch chips, toasted coconut (reserve a handful for sprinkling on top of the finished bars, if desired).
Add butter to a large saucepan over medium heat. Once it’s almost melted, add the remaining marshmallows and stir well to coat in the butter. Cook, stirring constantly, until almost completely melted. Remove from heat and stir in remaining 1 cup of butterscotch chips. Stir well until smooth and, working quickly, pour over bowl with cereal and stir to coat.
Pour mixture into the prepared pan and press into an even layer. Sprinkle coconut flakes on top, if desired. Cool for a few minutes before cutting into bars.
For best results, store the bars wrapped individually in plastic wrap for up to 2 days.