Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt and baking soda. In a large bowl, mix together butter, peanut butter and both sugars until fluffy. Add in egg and vanilla extract and mix until smooth.
Add in the flour mixture and mix until it is fully combined. Fold in the marshmallows. Use a cookie scoop to place the cookies onto a large ungresaed baking sheet at least 2-inches apart. Bake for 10-12 minutes.