Preheat the oven to 350 F. In a medium bowl, whisk together flour, salt, and baking soda. In a large bowl, combine butter with sugar until fluffy. Add in 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 place each one onto a large ungreased baking sheet at least 2-inches apart. Bake for 10-12 minutes.
Remove half of the cookies from the baking sheet. Place one marshmallow and some chocolate chips on each cookie on the baking sheet. Bake for 2-3 minutes. Remove from the oven, and sandwich with the remaining cookies.
