Line a 9x13-inch baking pan with wax paper and set aside.
In a large pot over medium-low heat, add the butter, peanut butter, and mini marshmallows.
Stir frequently and cook until the butter, peanut butter, and marshmallows are completely melted and smooth. Add in the vanilla and stir again.
Remove the pot from the heat and add the crispy cocoa rice cereal and peanut butter cups.
Stir until everything is well combined.
Pour the mixture into the prepared baking pan.
Using a lightly greased spatula or another piece of wax paper, gently press the mixture evenly into all four corners of the pan.
Press a few additional mini peanut butter cups into the top, if desired.
Place the pan in the refrigerator while you melt the chocolate.