In a medium saucepot, melt the butter over medium-low heat. Once the butter has melted, keep a watchful eye on it while whisking it. In about 5 minutes, it'll turn a deep amber color after foaming and fizzing for awhile. Remove it immediately from the heat once you see that amber color.
Immediately stir in the salt, vanilla, and 8 cups (1 pkg) of marshmallows and stir to combine. The residual heat of the saucepot should melt the marshmallows, but if it doesn't melt them all the way, turn the heat back to low and stir the marshmallow mixture constantly until melted.
Fill a large bowl with the Rice Krispy cereal and pour the marshmallow mixture into the bowl. Add in the remaining 2 cups of marshmallows and toss everything to coat. Once fully coated, spread into a greased 13x9" baking pan. To compact the cereal mixture into neat bars, grease the bottom of a glass cup with cooking spray and press down on the krispy treats until compacted.
Let set for about 1 hour at room temperature before cutting into bars.