Butter a 9 x 13 pan. Line with a strip of parchment paper. (this makes it easier to lift when done to slice)
In a large pot, melt butter over medium heat. Heat butter until, the milk solids begin to separate and darken. The butter is ready when you see dark flecks throughout the butter.
Once butter is browned, add brown sugar and stir.
Next, add in 2 of the 3 bags of marshmallows, stir until melted and smooth.
Immediately remove from heat and add pinch of salt and vanilla, stir. Quickly mix in the Rice Krispy Cereal.
Finally, add the last bag of marshmallows, and fold in.
Dump mixture into the buttered pan. Lightly spread into place, this will yield fluffy and light treats. DO NOT press them down firmly as this will make them too firm.
Allow treats to cool before cutting into 24 squares (4 squares x 6 squares). Store any leftovers in an airtight container.