As a note, I buy 1 large (16 ounce) bag and 1 small (10 ounce) bag of mini marshmallows! This makes it super easy.
Spray a 9x13 inch baking dish with nonstick spray. I like avocado spray.
First, measure out 1 cup of mini marshmallows and set them aside.
Place the butter in a large stock pot over medium heat. Let it melt and bubble until it begins to brown a bit.
Reduce the heat to low. Stir in the rest of the marshmallows. Stir with a spatula until the marshmallows melt. Make sure to keep this over low heat!
Once the marshmallows are melted, turn off the heat. Stir in the vanilla. Stir in the golden grahams, adding half and stirring, then adding the remaining half. Try to fold and stir until they are well combined.
Add the remaining 1 cup of marshmallows and the mini peanut butter cups. Work quickly but gently, stirring in the marshmallows and peanut butter cups.
Press the mixture into the greased pan. I use a sheet of parchment or wax paper to press the mixture into the pan well, getting into the corners and making sure the top is even.
Let the bars sit for at least 30 minutes before slicing. Serve!