Line a 9 x 9-inch pan with parchment paper and set it aside.
In a deep pot, melt your butter. Once melted, add the marshmallows and mix very well, until it melts down and combines with the butter. Remove from the heat.
Add the crispy rice cereal and mix until the cereal is mostly coated in the marshmallows.
Transfer to the lined pan and gently transfer the rice krispies mixture into it. Using your hands, lightly press down onto it. Let it sit at room temperature, before slicing into bars.
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Notes
TO STORE: Rice crispy treats should be stored at room temperature in a sealed container or ziplock bag. They will keep well for up to 4 weeks.TO FREEZE: Place leftovers in a shallow container and store them in the freezer for up to 6 months. Thaw them completely to room temperature before enjoying them.