Lightly grease a 9x9 pan with baking spray, or line with parchment paper. Set aside.
Brown the butter: In a large light-bottomed Dutch oven over medium heat, melt the butter. The butter will begin to foam, and then the foam will begin to subside and you’ll start to see tiny specks at the bottom of the pan, stir with a heat resistant spatula or wooden spoon and constantly stir and scrape so milk solids that are browning don’t stick. (These are what give brown butter it’s yummy flavor.)
1/2 cup unsalted butter
As soon as the butter turns chestnut brown and omits a nutty aroma, add in 6 cups marshmallows. Stir with a wooden spoon until melted then remove from heat and turn off the burner.
8 cups mini marshmallows, divided
Stir in the vanilla and salt. Then stir in the peanut butter.
2 tsp. vanilla extract, 1/2 tsp. salt, 3/4 cup creamy peanut butter (or cookie butter!)
Stir in the rice crispy cereal with a wooden spoon or rubber spatula. It will be sticky! Just keep stirring.
6 cups Rice Krispie Cereal
Add in the final 2 cups mini marshmallows and stir just until mixed. You want to leave tiny pockets of marshmallow.
Transfer the rice krispie mixture into the prepared pan. Gently press down using the back of the butter wrapper or a greased spatula. Be sure to lightly press instead of tightly compacting for best results.
Garnish with flaky sea salt (or better yet, melted dark chocolate!)
Let the treats cool at room temperature for 30-60 minutes. Be sure to cover them if leaving out for longer than an hour.
Lift the bars out of the pan and place on a cutting board. Cut into bars and enjoy!