Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt, and baking soda. In a large bowl, combine butter with peanut butter, brown sugar, and granulated sugar until fluffy.
Add in the egg and vanilla extract, and mix until smooth. Add in the flour mixture, mix until it is fully combined. Roll the cookie dough into balls and flatten slightly, and place marshmallows on each piece of cookie dough. Roll into a ball, covering the marshmallows completely.
Place the cookies onto a large ungreased baking sheet at least 2-inches apart. Bake for 10-12 minutes.
