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Melt butter in a large saucepan over low heat.
Add marshmallows and stir until completely melted.
Remove from heat.
(It would be perfect to be measuring out all of the cereal at this time, having it ready in a large bowl.).
Working quickly, add rice krispies cereal, and stir until the cereal is well coated.
Using a buttered spatula or waxed paper, press the mixture evenly into a greased 13 by 9 by 2 inch pan (or whatever pan you have on hand).
When the treats are cooled, cut into squares.
Serve, and enjoy!