Preheat oven to 350 degrees (use convection bake if you have it).
Using a stand mixer fitted with the paddle attachment or a hand mixer beat softened butter and brown sugar on high for 1-2 minutes. Scrape down the bowl once complete.
Add in milk, egg, and melted chocolate. Mix until combined. Scrape down the bowl once complete.
In separate bowl, whisk dry ingredients together (flour, cocoa powder, baking soda, salt).
Add dry ingredients to wet ingredients in 2-3 portions. Mix until combined, scrape down the bowl once complete.
Use a cookie scoop to scoop the cookies onto a greased cookie sheet (or a cookie sheet lined with parchment paper).
Bake for 8-10 minutes or until the top of the cookies just begins to crack.
Allow the cookies to cool completely before dipping them in the peppermint bark. Place the cookies in the freezer to have them cool faster.