Line a 9×13″ (or 9x9" for thick bars) baking dish with foil and butter. Set aside.
Melt butter in a large saucepan over medium-low heat and then add 7-8 cups marshmallows. Once they are almost melted, stir in the rice krispies cereal and remaining 2-3 cups of mini marshmallows. Mix until completely coated with the marshmallow mixture.
Add mixture to baking dish and press down with a spoon coated in butter or wax paper to compact and level out surface.
Let cool before cutting. I find using a plastic knife to cut them works best.
Notes
Rice krispies will keep for up to 3 days if stored in an airtight container or bag.Feel free to make fun variations for the holidays or special occasions. Just add food coloring, sprinkles or extras like M&M’s or chocolate chips.