Spray a 8x12 inch baking dish with cooking spray and set aside.

Melt butter in a large pot over medium-low heat.

Reserve 1 1/2 cups marshmallows and pour the remaining marshmallows into melted butter.

Stir constantly until marshmallows are almost melted then stir in vanilla extract.

Stir in crispy rice cereal and remaining marshmallows and gently stir until coated with melted marshmallow mixture.

Pour mixture into prepare baking dish.

Spray hands with cooking spray and press mixture evenly into baking dish.

Cool completely before cutting into squares.