Grease a 9x13" pan generously with butter or line with parchment paper.

Melt butter in a large pot over medium heat. Continue cooking until the butter starts to foam. Cook and stir with a wooden spoon until the milk solids at the bottom of the pan turn a golden brown color. The butter will have a nutty aroma.

Quickly stir in the bag of marshmallows and turn the heat to low. Cook and stir until the marshmallows are melted. Stir in the vanilla and salt.

Remove from the heat and add the crispy rice cereal. Stir until well coated with the marshmallow mixture.

Transfer to the prepared pan. Use your hands to press the cereal mixture gently into the pan. Don't press too firmly, or the bars will not be as soft.

Let the rice krispy treats cool for 30-60 minutes, then cut into bars.