Take a deep form 9 * 9 inches and line it with foil. Sprinkle a little with a cooking spray.
In a large saucepan with a thick bottom, melt the butter over low heat.
Pour 8 cups of mini-marshmallows and stir constantly.
Once the marshmallows have melted a little, add salt and crispy rice, stir quickly so that the marshmallows cover the crispy rice and remove from heat.
Put the cooked mass into a mold and smooth with a spatula.
Let cool completely and cut into squares.
Notes
So that the mass can be easily leveled in shape, moisten the spatula or fingers in water.