For the shortbread: Preheat the oven to 350 degrees F. Line baking sheets with parchment paper.
In a mixer, cream the butter and sugars. Add the melted chocolate and peppermint extract. On low, mix in the flour, cocoa, baking soda and salt. Mix until all ingredients are combined and the dough comes together. Mix in the cocoa nibs. Divide in half and place in the refrigerator for 30 minutes.
Roll each ball between two sheets of parchment paper to 1/4-inch thick. Cut with a 2-inch round cookie cutter and transfer the cookies to the prepared baking sheets. Bake for 12 to 14 minutes; the centers will feel a little soft but will harden as they cool.
For the mint: Wash the mint and pick the smallest, most sexy leaves (you'll need 24 plus some extra in case you mess up). Pat dry with a paper towel.
In a small microwave-safe bowl, sprinkle the gelatin over 1/3 cup cool water and let bloom. Melt in the microwave 20 to 30 seconds.
Hold the mint leaves by the stems and dip in the gelatin, one at a time. Shake off the excess. Press the leaves in the sanding sugar, coating both sides, and place on paper towels to dry.
Temper your chocolate: Place two-thirds of the chocolate in a microwave-safe bowl. Microwave on medium in 30 second intervals until fully melted, about 3 minutes. The temperature of the chocolate should be around 120 degrees F. Add remaining one-third of the chocolate and stir until fully incorporated. Keep stirring until the chocolate reaches 85 degrees F. You can test it by holding the chocolate covered spatula to your lower lip. When it no longer feels warm, the chocolate is ready. You can warm it SLIGHTLY, to 90 degrees F, to make it more fluid. This can be done by microwaving it in 2 second intervals. I am not joking - 2 seconds, no more! Keep the chocolate on a heating pad set to low if you are not going to use it for a while. Make sure the temperature stays between 85 and 90 degrees F. If it goes over, you will have to start the process again. And you DON'T want to do that.
To assemble: When the cookies are cooled, dip each cookie halfway into the tempered dark chocolate and set on a sheet of parchment. Press 1 crystallized mint leaf in the center of the cookie. The chocolate will act as a glue. Let the chocolate set and enjoy!