Spray a 9 x 13 inch pan with nonstick cooking spray, or rub with butter. Measure out Rice Krispies; set aside.
In a large pot, melt butter and 5 cups mini marshmallows over medium-low heat, stirring constantly, until mixture is completely smooth, with no discernible bits of marshmallow.
Remove pan from heat and immediately stir in cereal and remaining 2 cups mini marshmallows, until all cereal is coated with melted marshmallows. The mini marshmallows you add in will stay relatively whole.
Scrape the mixture into the prepared pan and press down lightly into an even layer. Let cool at least 10 minutes before cutting into squares. For best taste and texture, serve the same day.
Notes
Be sure to press down lightly for the best texture! Spray your hands with nonstick spray, or use a piece of wax paper, or the butter wrapper to prevent the mixture from sticking to your hands.
If desired, you can skip the extra 2 cups of marshmallows.
Storage: After cooling bars completely, store in an airtight container at room temperature for up a week. The bars will become a bit drier and harder as time goes on.