• Line an 8x8 baking pan with parchment paper leaving an overhang on 2 sides or alternately spray with cooking spray.
• In a large saucepan, melt the butter on medium-low heat until it turns a golden-brown color and has a nutty smell, about 5-7 minutes. It will foam up after coming to a boil so keep stirring and checking the color under the foam. Once it starts to turn color and you see brown bits, immediately go to the next step.
• Add the peanut butter and all but 1 cup of the marshmallows to the browned butter. Stir constantly until it's fully melted and combined.
• Remove the saucepan from the heat and stir in the vanilla and salt if using unsalted butter. Quickly stir in the rice krispies until combined. Let the mixture sit for 1-2 minutes and then stir in the reserved cup of mini marshmallows.
• Pour the mixture into the prepared pan and smooth the top with a spatula or your hands until it's even. Don't press too hard! If it's too sticky, spray the spatula or your hands with cooking spray.
• When you are ready to serve them, lift the whole block of rice krispie treats out of the pan using the parchment overhang. Slice them with a large knife.

Peanut Butter Glaze

• Combine the powdered sugar, peanut butter and 5 tablespoons of milk in a small bowl. Add additional milk by the teaspoon until you get a pourable consistency to drizzle over the rice krispie bars.