Preheat oven to 350 degrees and prepare a baking sheet with parchment paper.
In an electric mixer fitted with a paddle attachment, cream butter, sugar, brown sugar, and peanut butter. Mix until light and fluffy.
Add eggs and vanilla.
Add eggs and vanilla.
Fold in chocolate chips and marshmallows.
Using a medium ice cream scoop, scoop round balls onto cookie sheet and bake for 10 minutes. Remove cookies from oven and immediately add 5-6 marshmallows. Allow cookies to cool on the baking sheet.