Beat butter and brown sugar.
Add egg and yolk.
Beat in the peanut butter and vanilla extract.
Add the dry ingredients.
Add chocolate chips.
Preheat oven to 350ºF and line baking sheets with parchment paper.
Scoop dough into 2 oz portions.
Make an indentation in the center and lay in 3-4 marshmallows. Encase them with the dough so they do not show.
Flatten the best you can bc they won't spread.
Top with a few extra chocolate chips.
Bake 9-10 mins.
Cool.