Heavily butter a 13x9" baking dish. Set aside.

Combine butter and 5 c mini marshmallows in a large pot over medium heat. Stir continuously until melted. Take off heat.

Stir in vanilla and remaining marshmallows.

Quickly pour rice krispies into marshmallow mixture and stir gently stir with buttered large spoon.

Place the mixture into the baking pan and chill for a few minutes.