This recipe is reprinted with permission from Everything Chocolate: A Decadent Collection of Morning Pastries, Nostalgic Sweets, and Showstopping Desserts from America's Test Kitchen. It is based on the famous Girl Scout Thin Mints and produces a snappy, chocolate covered minty cookie that's perfect for a holiday treat and for gift giving. For the best results, the recipe recommends using couverture chocolate, a professional quality chocolate available for ordering online. These chocolate-mint cookies can be stored at room temperature for up to 2 weeks.
Whisk flour, cocoa, salt, baking powder and baking soda together in bowl and set aside. Using stand mixer fitted with paddle, beat coconut oil and sugar on medium-high speed until fluffy, about 2 minutes. Reduce speed to low, add the milk, egg, and vanilla, and beat until combined, about 30 seconds.
Slowly add flour mixture and beat until just combined, about 1 minute, scraping down bowl as needed. Divide dough in half. Form each half into 4-inch disk, wrap disks tightly in plastic wrap, and refrigerate until dough is firm yet malleable, about 45 minutes.
Adjust oven racks to upper-middle and lower-middle positions and heat oven to 350 degrees. Line 2 baking sheets with parchment paper.
Working with 1 disk of dough at a time, roll into 11-inch circle, about 1/8-inch thick, between 2 large sheets of lightly floured parchment paper. Remove top piece of parchment. Using 1 3/4-inch round cookie cutter, cut dough into circles; space circles 1/2 inch apart on prepared sheets. Gently reroll scraps 1/8-inch thick, cut into rounds, and transfer to prepared sheets.
Bake until very firm, 16 to 18 minutes, switching and rotating sheets halfway through baking. Let cookies cool on sheets for 5 minutes, then transfer to wire rack. Let cookies cool completely.
Melt two-thirds of chocolate and oil in medium metal heatproof bowl set over small saucepan filled with 1 inch barely simmering water, stirring often, until chocolate registers 118 degrees. Remove bowl from heat and slowly add remaining chocolate, stirring constantly, until chocolate registers 82 degrees, 15 to 30 minutes. Briefly return bowl to saucepan and heat, stirring often, until mixture reaches 90 degrees, moving bowl on and off the heat every 15 seconds to prevent overheating. To test for temper, dip tip of butter knife in chocolate and let sit for 10 minutes. Chocolate should harden and be glossy.
Line 2 baking sheets with parchment. Working with 1 cookie at a time, place cookie on fork and dip bottom of cookie in chocolate. Using offset spatula, spread chocolate over top of cookie, creating thin coating. Transfer cookie to prepared baking sheet and repeat with remaining cookies. Let cookies sit until chocolate sets, about 10 minutes, before serving.