Lightly grease a 9x13-inch baking dish with butter or nonstick spray.
In a large pot, melt 3 tbsp butter over medium-low heat. Once the butter has melted, add the entire bag of marshmallows, stirring constantly until they are completely melted and smooth.
Once the marshmallows are melted, stir in 2 tsp of vanilla extract and a pinch of salt.
Remove the pot from the heat and quickly fold in 6 cups of Rice Krispies cereal. Stir until the cereal is fully coated in the marshmallow mixture. Be sure to work quickly!
Transfer the marshmallow-cereal mixture into your prepared baking dish. Use a spatula (or your hands, lightly greased with butter) to press gently on the mixture evenly into the pan.
Allow the Rice Krispies Treats to cool in the pan for about 30 minutes at room temperature. Once cooled, cut them into squares and enjoy!