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. Use a cookie scoop to place the cookies onto a large ungreased baking sheet. Bake for 10-12 minutes.
Place half of the cookies onto a large baking sheet and top with candy bars and marshmallows. Bake for 2-3 minutes. Remove from the oven and sandwich with the remaining cookies.
