Grease a 9x13-inch pan with butter or nonstick spray and set it aside.

In a large saucepan over low heat, melt the butter. Add 12 ounces of mini marshmallows and stir until they melt into a smooth mixture.

Remove from heat, then quickly stir in the vanilla extract, Rice Krispies cereal, and chopped Oreo cookies. Add the remaining 8 ounces of mini marshmallows and gently mix to combine.

Transfer the mixture into the prepared pan. Use a gentle hand to press the mixture evenly. Allow the treats to cool to room temperature before cutting into squares and serving.