In a large pot, melt butter and just 8 C marshmallows on the stove over medium heat (make sure you reserve 1 C of the marshmallows for later!). Stir frequently while it melts to melt evenly. While this melts, line a 9x9 pan with wax paper (can also use a smaller or larger pan, I just prefer the thickness of a 9x9!)
Once marshmallows are almost entirely melted and smooth, turn off heat and stir in 1/2 packet jello mix and stir to combine!
Fold in rice krispies and the remaining 1 C marshmallows until incorporated! Once combined, immediately spread into your prepared pan.
Allow to cool/set for at least an hour, preferably more before cutting! I typically allow mine to set for a few hours for an easier, clean cut!
Notes
*Cover/store in an airtight container at room temperature for 2-3 days (if they last that long)!