Line an 8"x8" inch square brownie pan with aluminum foil. Spray the foil with nonstick cooking spray.
In a large pot over medium-low heat melt the butter. After it has melted, turn the heat down to low. Now we brown the butter, it will take about 8 minutes for the butter to brown. Stir frequently throughout this process. The butter will turn foamy and golden colored specks will appear on the bottom of the pan. Stir frequently to prevent the butter from burning. The butter will have a beautiful nutty aroma.
Once the butter has taken on a nutty color, stir in 8 cups of mini marshmallows.
Once the marshmallows have just melted stir in the kosher salt and vanilla extract.
Remove the pot from heat and stir in the Rice Krispies cereal. Mix until the mixture is completely covered in the marshmallow mixture.
Stir in the remaining 2 cups of mini mallows.
Spoon the mixture into the prepared 8x8 inch pan. Gently press the mixture into the pan (but don’t press too hard you don’t want the treats too compact. Sprinkle some sea salt over the rice krispie treats.
Let set for 30-60 minutes then remove the foil and lift the rice krispie treats from the pan. slice into squares. Store in an airtight container for 4-5 days.