In a large pot or medium-low heat, melt the butter. Add the marshmallows and stir until melted. Reduce heat to low and stir in the cereal.
Pour mixture into prepared pan and press into even layer. Let cool. Cut into 2-inch squares and serve. Store in airtight container and room temperature for up to 4 days.
Notes
If using unsalted butter, just add pinch or two of salt to the butter when melted.