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.
Roll the dough into small 1-inch dough balls and place them on a well-greased cookie sheet (you can also line the cookie sheet with parchment paper). Each cookie should be at least 2 inches apart.
Bake for 8-10 minutes or until the top of the cookies just begins to crack.
Remove them from the oven and use a teaspoon measuring spoon to add a 'thumbprint' in each cookie. Do this immediately when they come out of the oven.