Melt the butter in a saucepan over medium-low heat.

Remove 1 cup of marshmallows from the bag and set aside. Add the remaining marshmallows to the melted butter and stir over low heat until the marshmallows have melted and there are no lumps.

Remove the marshmallow mixture from the heat and stir in the vanilla extract.

Place the Rice Krispies cereal in a large bowl. Pour the marshmallow mixture over the Rice Krispies and stir until fully incorporated. Stir in the reserved 1 cup of marshmallows.

Grease a 9x13 pan or baking sheet with cooking spray, or add parchment paper.

Pour the Rice Krispies mixture onto the pan and gently press to spread it out. Don't press too much, or the treats will become hard later.

Allow the Rice Krispies Treats to cool and then cut into squares. Keep them covered in an airtight container and they should taste good for 2-3 days.