Add butter to a large non-stick pot and cook over medium heat for 2-3 mins. Stir occasionally, until butter turns golden brown and smells nutty.
Remove the skillet from the heat and add 2 bags (12 cups) of marshmallows. Stir to start to melt the marshmallows so they get fluffy. The marshmallows shouldn't be fully melted.
Once the marshmallows are fluffy (but not fully melted), add the cereal, butterscotch chips, and salt to the pot. Stir until cereal is evenly coated.
Fold in the last half bag (3 cups) of marshmallows. Don't let them melt all the way. The marshmallows should still be intact!
Transfer to a 9x13 inch pan (or divide between 9x13 inch pan and 8x8 inch pan if you want them thinner). Place a piece of parchment paper on top & press hard to make sure the rice krispie treats are even and compact.
Let cool & cut into bars with a super sharp knife.