Melt butter and marshmallows in a large saucepan over medium-low heat, stirring frequently. Remove from heat when almost melted and stir until smooth. Alternatively, this can be done in the microwave (see notes).
Stir in cereal.
Spread into pan and let set.
Cut into squares and enjoy!
Optional: Dip squares into melted white chocolate and let set.
Notes
To melt the marshmallows and butter in the microwave, heat for 45 seconds then stir. Continue microwaving in 15-20 second increments, stirring between each, until smooth.In place of lining the pan with parchment paper, you can butter the pan.Wax paper can be used to gently press the marshmallow cereal mixture into the pan without it sticking to your hands. Do not press firmly!Let the bars set completely before cutting or they may fall apart.