3 Tablespoons butter
40 marshmallows (I like campfire brand but for some reason they are hard to find. I hate Jet Puffed)
6 cups cereal (cocoa rice krispies or plain)
Coat 13x9x2 inch pan with cooking spray. Melt butter over low heat. Add marshmallows and stir until completely melted. Removed from heat and stir in cereal. I like to use a cooking spray-coated metal spoon to scoop the cereal out and press into the pan. Cool and eat.
I used Halloween Cocoa Rice Krispies. They had Lucky Charms-esque marshmallow bats and ghosts in them, which was cute, but I thought they might have a weird texture in the treats so I picked them all out (almost 3/4 of a cup worth!) and sprinkled just a few on top.