Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt, and baking soda. In a large bowl, mix together butter and sugar until fluffy. Add in the 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 at least 2-inches apart. Bake for 10-12 minutes.
Remove from the oven and place half of the cookies onto a large baking sheet. Spread with peanut butter and top with chocolate bars and marshmallows. Bake for 2-3 minutes. Remove from the oven and sandwich with the remaining cookies.
