Grease a 9x13 casserole pan with butter, set aside.
In a large, light colored, heavy bottomed pot, like an enameled dutch oven, melt butter over medium heat.
Once melted, add 2 bags of the marshmallows and stir until marshmallows are melted and the mixture is completely melted and smooth.
Remove from heat, then immediately stir in the vanilla extract and salt. Fold in crispy rice cereal, moving quickly and then fold in remaining bag of mini marshmallows.
Pour mixture into prepared pan. Using a buttered spatula, lightly press them into pan. Do not press them down too much, just spread them out. If you press them down too much they will become very dense and not as fluffy.
Allow treats to rest for at least 1 hour at room temperature too cool. Slice into squares or cut into shapes and store any leftovers in an airtight container for up to a week.
Notes
When stored in an airtight container, they will stay fresh for up to a week. You can also freeze them wrapped individually with plastic wrap for up to 3 months.