Preheat the oven to 350 degrees.
Using a stand mixer fitted with the paddle attachment or a hand mixer, cream the butter and brown sugar on high for 1 minute. Scrape down the bowl once complete.
Add in milk, egg, vanilla extract and melted chocolate. Mix until combined. Scrape down the bowl once complete.
In a separate bowl, whisk the dry ingredients together (flour, cocoa powder, baking soda, salt).
Add the dry ingredients to wet ingredients in 2-3 portions. Mix until combined, then scrape down the bowl once complete.
If using a metal muffin tin, grease it well with baking spray. If using a silicone muffin mold, no need to grease it.
Use a 2-inch cookie dough scoop (or roll the dough into a 2-inch ball with your hands) to measure out the cookie dough. Place one scoop in each muffin well (pictured above).
Bake for 16-20 minutes, or until the edges begin to crisp.
Remove them from the oven and immediately add an indent to the center of each cookie using a tablespoon measuring spoon (pictured above).
Allow the cookie cups to cool to room temperature before adding the chocolate cream filling.