First, start by creaming together the butter and sugar in a stand mixer or in a bowl if using a hand mixer.
Add in the egg, peanut butter and vanilla extract while mixing.
In a separate bowl, whisk together the flour, baking powder and salt.
Add the flour mixture about a 1/4 cup at a time to the wet ingredients while mixing. Make sure to stop and scrape the sides of the bowl as needed.
Fold in the marshmallows and dark chocolate chunks.
Preheat the oven to 350 degrees F.
Line a baking sheet with parchment paper.
Use a 1 inch scoop to scoop the cookie dough out of the bowl and place them onto the parchment paper lined baking sheets.
Bake in a preheated 350 degree F oven for 10-12 minutes.
Let the cookies cool on the baking sheet for at least 10 minutes before transferring to a cooling rack to finish cooling.
Finally, serve your peanut butter marshmallow cookies with a cold glass of milk if desired and enjoy!