Spray a 9-by-13-inch baking dish with nonstick cooking spray. Set the dish aside.
Add the butter and 16 cups of miniature marshmallows to a large saucepan or stockpot.
Melt the butter and marshmallows over medium heat. Stir continually.
When the butter and marshmallows have melted, pour in the salt and vanilla. Stir to combine.
Remove the saucepan or stockpot from the heat.
Pour the Rice Krispies into the marshmallow mixture and blend until the cereal is thoroughly coated.
Stir in the remaining 4 cups of miniature marshmallows.
Spoon the mixture into the prepared baking dish and press it firmly down into the pan.
Allow the mixture to set in the pan for 30 minutes. Slice it into squares and serve. Enjoy!
To get 20 cups of miniature marshmallows, buy 3 10-ounce bags of miniature marshmallows.
You may substitute larger marshmallows for miniature marshmallows in this recipe. Use a digital scale to measure out 680 grams (the weight of 20 cups of miniature marshmallows).
Store your homemade Rice Krispies treats in an airtight container at room temperature for 3-5 days. Do not store in the refrigerator.