Place mint leaves and 1 Tablespoon of sugar into bowl of a small food processor. Pulse several times until mint is minced well into sugar. Set aside to marinate for a few minutes.
In a medium size bowl, sift together the flour, salt and baking soda. Set aside.
Place butter, both sugars and mint/sugar mixture into stand mixer bowl fitted with the paddle attachment. Beat on medium speed until light and fluffy (about 3-5 minutes.) Add egg beat on medium speed until well combined. Switch speed to low and slowly add the flour mixture, beating only until the streaks of flour have disappeared. Stir in chopped chocolate.
Cover the bowl and refrigerate for at least six hours (or overnight, preferably).
Preheat oven to 375 F. and line a baking sheet with parchment paper. For each cookies, measure out a heaping tablespoon of dough (mine weighed about 2oz each).
Place cookies on prepared baking sheet, leaving about 1-inch of space between each (they don't spread very much). Bake until light golden, about 9-13 minute. Remove cookies from oven and let them rest on the sheet for about 10 minutes before transferring to a cooling rack, or enjoy while they're still warm & fragile.