Melt the butter and 2½ packages of the marshmallows over low heat in a Dutch oven. Add the pinch of salt and vanilla extract.
Add the cereal and the rest of the marshmallows and stir to coat.
Turn it out into a 9 x 13 pan lined with greased foil. DON'T PRESS FIRMLY. Press very gently just to spread cereal treats into the corners of the pan. Let cool. Slice into large squares.