Line a 9x13-inch baking dish with foil, then coat with non-stick cooking spray; set aside.
In a large saucepan or stockpot, melt the butter and 16 cups (680 grams) of the miniature marshmallows over low heat until the marshmallows are just melted. Stir in the salt and vanilla to combine. Remove the pan from the heat, add the Rice Krispies and mix until it is completely and evenly coated with the marshmallow mixture. Stir in the additional 4 cups of miniature marshmallows.
Turn the mixture out into the prepared pan and, using a wooden spoon or spatula that has been coated in non-stick cooking spray, press the mixture firmly into a packed, even layer. Allow to set at room temperature for at least 30 minutes. Using the foil, remove the treats from the pan and slice into squares. Leftover rice krispie treats should be stored in an airtight container at room temperature for up to 3 days.
Marshmallows: You will use three (10-ounce) bags of miniature marshmallows for this recipe.
Omitting Extra Marshmallows: You can omit the extra marshmallows that get stirred in at the end if you wish. You do not need to make any adjustments to the recipe.
Using Marshmallow Fluff: You can substitute four (7.5-ounce) jars of marshmallow fluff for the 16 cups of miniature marshmallows that are called for in the recipe. Heat the marshmallow fluff with the melted butter over low heat and proceed with the recipe as directed.
Mix-Ins & Flavors: See the post above for suggestions on mix-ins and additional flavors.
Storing + Shelf Life: These are best within 1 day of making them, but you can store them in an airtight container at room temperature for up to 3 days. To keep them ultra-fresh, wrap them individually in plastic wrap and place in an airtight container.
Freezing: If you want to freeze them, wrap them individually in plastic wrap and place in a freezer ziploc bag, and freeze for up to 2 months. Allow to thaw at room temperature.