Melt marshmallows in a large microwave-safe bowl, butter, and salt in a large mixing bowl. About 2 minutes- checking every 20 seconds or so.
Mix in the cereal.
Press into a 9x13 pan sprayed with non-stick spray and add any toppings.
Cool and serve.
Notes
*Careful! The bowl and melted marshmallows will be hot!!*-A tip for pressing the crispy rice into the pan is to wet your fingers after every few pats. You should be able to have a nice firm top.