Line a 9x13" baking dish with parchment paper or lightly grease with butter.
Brown the butter - Heat butter in a large saucepan over med-low heat, stirring occasionally, until deep golden with dark specks on the bottom of the pan and fragrant. It will melt, then foam, before turning deep golden. Watch it closely! Once it is browned, it will burn quickly if left to continue cooking.
Turn heat down to low and add mini marshmallows. Stir until marshmallow have melted.
Take mixture off heat and add Rice Krispies. Stir until evenly combined, then press into prepared baking dish with buttered hands. Add sprinkles on top while still warm and gently press in. Slice and eat!