Melt butter in a large saucepan over medium heat. Set 1 cup of marshmallows to the side.
Lower the heat and add in mini marshmallows. Slowly melt until completely melted.
Remove from heat, add in vanilla and stir into marshmallows. Add in rice krispie cereal until nicely coated. Add in remaining 1 cup of mini marshmallows and gently fold into the cereal mixture.
Pour rice krispie marshmallow mixture into prepared baking dish. Gently press the mixture until it evenly spread in the dish.
Allow the treats to cool for an hour before enjoying.