Add 1 cup of water to your inner pot.
Insert your steamer basket or bowl with trivet into the Instant Pot. Add the bag of marshmallows.
PRESSURE COOK for 0 minutes. When it's done immediately release the pressure.
Stir in 7 cups of rice crispy cereal and the optional butter. Only stir butter into a bowl, it will melt through a steamer basket.
Pour the mixture into a casserole dish and use parchment paper to compress. Refrigerate for an hour to set and enjoy!