In a large microwave safe bowl, heat the marshmallows and butter in the microwave, 30 seconds at a time until melted and smooth. (more marshmallows make a stickier bar, so use 5 1/2 or 6 cups accordingly.)
Remove from the microwave and stir in the vanilla.
Let cool slightly to prevent the chocolate from melting too much and then stir in the cereal, Oreo's and the Peanut butter cups.
Press into the bottom of a well greased 9x13 pan.
Let set and then slice to serve!
Notes
You can play around with the candy additions as you like and try new flavors.