Preheat your oven to 350°F (175°C). Line your baking sheets with parchment paper to prevent sticking.
In a medium bowl, whisk together the all-purpose flour, cocoa powder, baking soda, and salt. Set aside.
In a large mixing bowl, beat together the unsalted butter and vegetable shortening until creamy. Gradually add the dark brown sugar, mixing until well combined and fluffy.
Add the egg and vanilla extract to the butter mixture, beating until fully incorporated.
Gradually add the dry ingredients to the wet mixture, alternating with the milk. Start and end with the dry ingredients, mixing until just combined.
Using a cookie scoop, drop rounded tablespoons of batter onto the prepared baking sheets, leaving enough space between each for spreading.
Bake in the preheated oven for 10-12 minutes, or until the edges are set. The cookies should look slightly underbaked in the center. Remove from the oven and let them cool on the baking sheets for a few minutes before transferring them to a cooling rack.