Line an 8x8" square pan with foil and spray with nonstick cooking spray. Set aside.

In a large pot, melt the butter over medium-low heat. Once melted, turn heat to low and begin to brown the butter — stir frequently until it foams and has brown bits on the bottom (about 8 minutes). It will have a nutty color and aroma when ready.

Once butter is browned, stir in 8 cups of mini marshmallows until just melted. Then add kosher salt and vanilla extract.

Remove the pot from the heat and stir in the crispy rice cereal until the entire mixture is coated. Stir in the remaining 2 cups of mini marshmallows and spoon into the prepared pan. With slightly wet hands, press gently into the pan (do not press too hard). Sprinkle with sea salt and sprinkles if using.

Allow to set for 30-60 minutes. Remove the foil, lift the treats from the pan, slice into squares and package individually if desired.