Spray a 9x13-inch baking pan (I use a pyrex 9x13) with nonstick cooking spray.
Melt butter in a large pot over medium-low heat. Continue to cook the butter, swirling the pan occasionally. Continue to swirl the pan until the butter develops a nutty aroma and brown bits start to form at the bottom.
Once the bits are amber in color and the butter is browned, add the vanilla, marshmallows, and salt to the pan. Reducing the heat to low, stir constantly until the marshmallows are completely melted. Remove from heat.
Add the rice cereal and stir with a rubber spatula or wooden spoon until the cereal is evenly coated. Transfer the mixture into the prepared baking pan, pressing the mixture gently with your spatula, wooden spoon, or your fingers with a layer of parchment paper. Don’t be too aggressive pressing the mixture into the pan. You want to be firm enough to bind the mixture together, but not so firm that you compress it super solidly. The more tightly you pack it in, the more solid the end result will be once set. Be a little gentle so they stay chewy once set.
Allow to cool to room temperature, about 1 hour. Cut into squares before serving. Squares can be stored in an airtight container at room temperature for up to 2 days.
