Place a large pot over medium heat and add the butter. Once melted, pour in the jet puffed marshmallows and stir until melted and smooth.
Remove from heat. Stir in the vanilla and Golden Grahams until evenly combined. Add the frozen mini marshmallows and peanut butter cup pieces and stir together.
Transfer the mixture to a 9-inch x 13-inch baking dish and spread into an even layer. (I do this with my hands after spraying them with cooking spray, but be careful! The mixture is quite warm.)
Set aside to cool for about 1 hour. Cut into 12 squares, serve, and enjoy!
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Notes
Nutritional information will vary based on how big you cut the bars. I usually get 12 or 15 from this recipe.
Freezing the mini marshmallows and peanut butter cups (after cutting) helps prevent them from melting into the warm cereal mixture when you're stirring.