Coat a 9"x9" baking pan with non-stick spray and set aside.
In a large, heavy bottomed pot, heat butter over medium heat. Stir often until butter is melted and begins to toast lightly. The color should turn a golden brown and give a nutty smell. This should take about 5 to 8 minutes.
Add the marshmallows to the browned butter (reserve 1 cup for the end). Stir until marshmallows are melted and smooth.
Remove from heat and stir in the vanilla. Pour in the puffed rice cereal and sea salt and stir to coat. Finally, stir in the remaining 1 cup of mini marshmallows and fold into the treats. Quickly pour this mixture into the prepared pan and gently press down. *use a sheet of parchment paper on top of the treats to keep your fingers clean while pressing down
Let the treats cool before slicing into squares and enjoying!