In a large bowl, whisk the flour, baking soda and salt together. In a stand mixer, cream the butter, peanut butter, sugars and vanilla on medium speed until light and fluffy, scraping the bowl as needed. Add the egg and mix again for another minute.
Reduce mixer to low and add the flour mix. Mix until just barely combined then add the chips and marshmallows and mix in. Chill the dough for 1 hour.
Preheat oven to 350 and line baking sheets with parchment. Scoop dough with a cookie scoop and bake cookies for 9-12 minutes until lightly browned on the edges and barely set in the center. Cool on sheet for 5 minutes, then transfer to a rack to fully cool.