Line a 9x13 inch baking pan with parchment paper and butter it thoroughly, including corners and sides to prevent sticking.
3 tablespoons butter
Heat butter in a large saucepan over low heat until melted. Add marshmallows and stir constantly for 3-4 minutes until smooth and lump-free.
3 tablespoons butter, 10 oz marshmallows
Remove pan from heat. Stir in vanilla extract and salt for 10 seconds until evenly mixed, working quickly before mixture cools and thickens.
1 teaspoon vanilla extract, 0.25 teaspoon salt
Add all 6 cups of Rice Krispies cereal to the marshmallow mixture and fold gently with a buttered spatula until every piece is coated, working fast to prevent setting.
6 cups Rice Krispies cereal
Scrape the mixture into the prepared pan and press evenly with buttered hands or foil without compacting tightly. Let cool at room temperature for 30-45 minutes before cutting into 12 squares with a sharp, buttered knife.