Line an 8 inch square pan with foil and spray foil with cooking spray or rub with some extra softened butter.
Melt butter in large (3 quart) non-stick sauce pan over medium-low heat. When butter is melted, add marshmallows and stir until melted. Stir the melted marshmallow mixture constantly, for about a minute after it is fully melted. Don’t let it burn.
Remove from heat and stir in vanilla and cereal. Stir until cereal is coated with marshmallow. Using a sheet of parchment paper or greased foil, press mixture evenly and firmly into the prepared pan. Let cool completely at room temperature. When fully set, lift from pan by grasping foil. Set on a cutting board and slice into large squares using a big chef’s knife.
This will give you 9 very nice sized marshmallow treats. For a double batch, double the ingredients and use a larger pot and a 13x9 inch pan.