Heat a large pot under medium heat and add butter until it is completely melted.
Lower heat to medium low. Add mini marshmallows, vanilla extract and peanut butter. Stir continuously until mixture is combined and marshmallows dissolve.
Remove from heat and add Rice Krispies cereal. Fold them into the sticky sauce.
Grease a 9' x 9' baking dish and fill it with sticky Rice Krispie cereal. Avoid pressing down on Rice Krispie Treats because we don't want to pack it in – it will make it hard to cut into pieces.
Let it sit for 30 minutes. Cut into 9 square pieces.
Serve and enjoy!
Notes
Alternatively, a 9′ x 13′ baking dish can be used as well. It will yield thinner pieces of Rice Krispie Treats.