Using a whisk combine the flour, cocoa powder, baking soda, and salt in a medium bowl. Set aside.
1¼ cups Flour, ½ cup Cocoa Powder, 1 teaspoon Baking Soda, ¼ teaspoon Salt
Using an electric mixer combine the softened butter, sugar, and dark brown sugar on medium-high speed until the mixture is light and fluffy, about 2 to 3 minutes. Scrap the sides of the bowl. Add the lightly beaten egg and vanilla extract and mix until incorporated, scraping down the sides of the bowl as you go.
8 tablespoons Unsalted Butter, softened to room temperature, ½ cup Sugar, ½ cup Dark Brown Sugar, 1 large Egg, lightly beaten, 1 teaspoon Pure Vanilla Extract
With the mixer on low, add the flour mixture to the butter mixture. As the mixture comes together, gradually increase the speed, mixing until everything is incorporated. The cookie dough will be sticky. Add the milk and mix well. Cover the bowl and refrigerate for at least 1 ½ hours and up to 2 days. Trust me when I say don't skip the chilling.
2 teaspoons Milk (I used 2%)
Remove the cookie dough from the refrigerator while you preheat the oven to 350℉ and line bakign sheets with silicone mats or parchment paper.
Using a cookie scoop, scoop the dough, roll in granulated sugar if desired, and place it onto the prepared baking sheets. Bake for 10 to 13 minutes.
½ cup Granulated Sugar, optional
Cool cookies for 1 minute and gently place an unwrapped Andes mint on top of each cookie. Let sit for about a minute then swirl the melting mint with an offset spatula. A knife will work but using the spatula is easier.
30 Andes Mints
Move cookies to a cooling rack and cool completely, allowing the mint swirl frosting to set.
Store in an airtight container for up to 3 days on the counter.