Line a 9-inch square baking dish with parchment paper and set it aside.

In a large saucepan, melt the butter over medium heat. Once melted, add all but 1 cup of the marshmallows. Lower the heat and stir continuously until the marshmallows are completely melted.

Remove the saucepan from the heat and stir in the vanilla extract. Add the rice krispies cereal and the remaining cup of marshmallows. Gently mix until everything is well combined.

Transfer the mixture into the prepared baking dish. Lightly press it down with your fingertips until it's evenly spread. Be gentle to avoid making the treats too dense.

Let the treats set for about an hour. Once set, cut into bars and enjoy!