1. Preheat oven to 325 degrees Fahrenheit. Line two baking sheets with parchment paper.
2. Combine flour, cocoa, baking soda, and salt in medium bowl. Set aside
3. In a separate (larger bowl), cream butter, brown sugar, and granulated sugar with hand-held mixer. Add egg and vanilla extract until thoroughly combined.
4. Gradually add dry ingredient mixture until thoroughly incorporated.
5. Drop equal-size scoops of dough evenly apart on sheets. Using an ice cream scoop really helps! Place in oven and bake approximately 17 minutes until surface is a bit wrinkly and cookies are only slightly soft to touch.
6. Transfer cookies from sheets to cooling racks. Cool thoroughly. If not, you will be very sorry.
7. In a medium bowl, beat 2 cups of powdered sugar with butter until creamy. Add salt and mint extract until thoroughly combined.
8. Add remaining (2 cups) of powdered sugar and milk, mixing well.
9. When your cookies have cooled sufficiently, place approximately 1 tablespoon of frosting on underside of one cookie. Top with another cookie. It's really not that hard.
10. Repeat for as many cookie sandwiches you desire. Consume any excess frosting as a 'snack' to tide you over before a cheeseburger dinner.