Lightly grease a 9”x13” pan with butter or cooking spray.
Set aside two cups of marshmallows.
Melt butter over low heat in a large pot.
Once the butter is melted, add all but the two cups of marshmallows, vanilla extract and salt. Stir with a greased spatula until marshmallows are completely melted.
Once mixture is smooth, remove the pot from the heat and add the full box of rice crisp cereal and additional two cups of marshmallows and stir with a greased spatula until well combined.
Pour the mixture into the lightly greased pan and, using either greased waxed paper or the butter wrapper, press and spread the mixture gently into an even layer.
Let cool at room temperature for 30-60 minutes.
Once cool, flip uncut treats onto a cutting board or serving platter and using serrated bread knife cut into uniform squares.