TO MAKE THE COOKIES:
1. in an electric stand mixer fitted with the paddle attachment, add the butter and brown sugar and mix on medium for about 4 minutes, or until well blended. With the mixer on low, add in the egg yolk, vanilla and peppermint extract and run for 2 minutes more or until completely blended. Scrape down the sides and bottom of the bowl to make sure everything is well incorporated.
2. In a medium bowl, whisk together the flour, cocoa powder, salt and baking powder. Add this to the butter mixture and run on low for 2 minutes, or until everything is combined and the dough has come together. Separate the dough into two hunks and shape each into a log with about a 2-inch (5-cm) circumference. It’s a little messy, but you can do it. Grab two pieces of plastic wrap, place a dough log on each, wrap tightly and roll the logs on the counter to round them out a bit. Place in the freezer for 1 hour or until firm.
3. Preheat the oven to 350°F (177°C) and cover several baking sheets with parchment paper. Place the sparkling sugar in a pie plate or casserole dish. Set aside.
4. Once the logs are firm and chilled, roll them in the sparkling sugar to coat and then cut 1⁄2-inch (1.3-cm) thick slices using a sharp knife. Place them on the baking sheets, leaving 2 inches (5 cm) between cookies. Place the cookies in the freezer for 10 minutes or the fridge for 20 minutes.
5. Bake one sheet at a time in center of the oven for 14 minutes. Let them cool on the baking sheet for 10 minutes and then transfer to a rack to finish cooling.
TO MAKE THE COOKIE COATING:
1. In a medium, heat-safe bowl add the dark chocolate and set over a medium saucepan of simmering water. Do not let the bowl touch the water. Stir frequently until melted and smooth and then turn off the heat. In another medium, heat-safe bowl add the white chocolate and set over a medium saucepan of simmering water.
Do not let the bowl touch the water. Do not let the water come to a boil. Stir frequently until melted and smooth and then turn off the heat.
2. Dip the tops of the cookies in the dark chocolate, letting the excess chocolate run back into the bowl. Drop some drops of white chocolate over the top and use a toothpick to swirl white chocolate circles into the dark chocolate.
Sprinkle the outer edges of the cookies with the crushed peppermint candy. Let the chocolate set before serving or storing.