Whisk together flour, salt, baking powder, and soda; Set aside.
In a double boiler over simmering water, melt 1 stick butter with the chocolate. Whisk until smooth. Set aside to cool.
When chocolate cools to room temperature, in the bowl of a stand mixer or with a hand held mixer, add the brown sugar. Add egg and mix until combined. Slowly add the flour mixture until well combined, stopping to scrape the sides if needed.
Cover with plastic wrap and cool for at least one hour.
Preheat oven to 350 degrees F. Line baking sheets with parchment paper.
Remove cookie dough from fridge. Scoop out heaping tablespoons onto baking sheet. (The dough will be hard to scoop). Bake for 10-12 minutes.
Cool cookies on baking sheets for 5 minutes then move to a wire rack. When the cookies have cooled to a manageable temperature, use a sharp knife and safety skills to slice the cookies in half. Let cool completely.
Meanwhile, make the filling. Cream together the 1 stick butter with the powdered sugar. Add the marshmallow fluff and creme de menth.
For more minty flavors, add Andes mint chips to the dough, or brush the cooling cookie halves with creme de menth, or just add more liquor to the filling. I won't tell if you don't.