Line a 9x13 inch baking pan with parchment paper or wax paper, leaving an overhang on the sides for easy removal.

In a large saucepan, melt butter over low heat. Once butter is melted, add marshmallows and stir continuously until completely melted.

Remove the saucepan from heat. Stir in the vanilla extract.

Add the Rice Krispies cereal to the marshmallow mixture. Stir gently until the cereal is evenly coated.

Pour the mixture into the prepared baking pan. Using a buttered spatula or your hands (lightly greased with butter or cooking spray), gently press the mixture into the pan. Do not pack too tightly, as this will make the treats hard.

Let the treats cool and set for at least 30 minutes to 1 hour.

Lift the treats out of the pan using the parchment paper overhang. Cut into squares and enjoy!