Preheat the oven to 350 F. Line 2 baking sheets with parchment paper or silicone baking mats.
Microwave the butter in a large, microwave-safe bowl for about 40 seconds. Butter should mostly melted.
Whisk the butter until it is completely melted.
Add the sugars; mix until well-combined.
Stir in vanilla and egg until incorporated.
Add the flour, cocoa powder, baking soda, and salt. Please read the recipe note about properly measuring flour.
Mix dough until just combined. Dough should be soft and a little sticky but not overly sticky.
Stir in chocolate chips.
Scoop out 1.5 tablespoons of dough (medium cookie scoop) and place 2 inches apart on baking sheet.
Bake for 7-10 minutes, or until cookies are set. They will be puffy and still look a little underbaked in the middle.
Gently press an unwrapped candy into the center of each cookie.
Let it sit for about 5 minutes to soften.
Carefully spread the melted mint over the cookie.
Remove to a wire rack to finish cooling.
Notes
Butter - I use salted butter. If you're using unsalted butter, increase the total salt to 1/2 teaspoon.
All-purpose flour - It's important to properly measure the flour. Too much flour will give you a crumbly dough or cookies that don't spread. To properly measure the flour, weight it or sift or stir it to break it up. Lightly spoon into a measuring cup and level.
Baking the cookies: It's important to slightly underbake the cookies. Slightly underbaking the cookies helps to keep the soft.