Melt butter and peanut butter together over medium heat in a large pot.
Once melted add marshmallows but reserve 15 marshmallows to add later.
Continue stirring to keep the bottom from browning.
Once the marshmallows have fully melted add the Rice Krispie's one cup at a time stirring each time.
Stop at 5 cups and assess if your treats are wet enough. If you want them a bit drier add another cup.
Once mixed add the 15 reserved marshmallows and stir.
Pour out into a parchment lined or buttered 9x13 dish.
With wet hands press the mixture into the dish gently. You don't want to compact the Rice Krispie's too much or they'll get dense.
Let rest for 30 minutes then slice and serve.