Butter the bottom and sides of a 9X13-inch pan. Set aside.
In a large saucepan, melt the butter over low heat and continue cooking as it foams and bubbles. Cook until the butter smells caramelly and fragrant and the small bits of butter solid on the bottom of the pan start to turn golden brown, 3 to 5 minutes (watch closely as brown butter can go from golden to burned quickly!).
Immediately add the marshmallows and cook over low heat, stirring constantly, until mostly melted with a few small lumps remaining.
Add the vanilla and salt and stir to combine.
Remove the saucepan from the heat and stir in the cereal until evenly combined.
Scrape the mixture into the prepared pan lightly press into an even layer with lightly greased hands. Cool completely at room temperature.
Cut into squares and serve.
Notes
Amount of Rice Krispies: For slightly gooier rice krispie treats, cut the amount of rice krispies to 8 or 8 1/2 cups.