Grease a 9 X 13 inch baking dish and set aside.
Put marshmallows, peanut butter, butter and vanilla extract in a large microwave proof bowl (or in a pot on the stove). Microwave for 1 minutes and then in 30 seconds increments and stir until smooth. Add Rice Krispie cereal and gently fold. Some people like a more chewy treat so you can add as much or as little cereal as you want!
Add Reese's Pieces and fold again.
Dump into prepared pan and press flat. Let cool and then serve!