Preheat oven to 350°F. Spray a 24-count mini muffin pan well with non stick cooking spray and set aside.
In a large mixing bowl, whisk together the flour, cocoa powder, baking soda, and salt.
In the bowl of a stand mixer fitted with the paddle attachment, or in a large mixing bowl using a handheld mixer, cream together the butter, brown sugar, and granulated sugar for 1-2 minutes. Then, mix in the egg and vanilla extract one at a time until well combined.
Slowly add the dry ingredients to the wet ingredients and mix until just combined, scraping down the sides of the bowl as needed.
Evenly distribute the cookie dough between all 24 cavities of the mini muffin pan (a little over 1 tablespoon for each one). Bake at 350°F for 11-13 minutes or until the tops of the cookie cups are set.
Remove from the oven and use the back of a measuring teaspoon to gently press in the center of each cookie cup. Allow to cool in the pan for five minutes, then remove from the pan and transfer to a wire rack to finish cooling.