Line a 9 x 13-in pan with parchment paper and lightly grease with non stick cooking spray. Set aside 2 C. mini marshmallows.
In a large pot with a light bottom so you can monitor the color, melt the butter over medium heat (you can save the butter wrappers; you'll use them later for pressing the mixture into the pan). Swirl the pan occasionally to be sure the butter is cooking evenly.
As the butter melts, it will begin to bubble and foam, and the color will progress from bright yellow to gold to finally a toasty-brown. Once you smell that nutty brown butter aroma, take the pan off the heat.
Off the heat, add the remaining marshmallows, vanilla, and salt.
Place the pot back over medium heat and stir the mixture until the marshmallows are completely melted. Remove the pan from the heat and add cereal, I like to do this step in a large bowl instead. Stir until evenly combined.
Add the 2 C. of reserved mini marshmallows and stir until they are softened and partially melted. Don't overtax; you want pockets of marshmallow goo. Transfer the mixture to the prepared pan and, using damp fingers or a spatula sprayed with cooking spray, gently press the mixture into an even layer in the prepared baking pan. Let cool at room temperature for at least an hour.
Use the parchment overhang to lift the treats onto a cutting board, then cut treats into squares. Enjoy!