Preheat oven to 350°F. Prepare a 24-count mini muffin pan with non stick cooking spray
In a large mixing bowl, fully stir the flour, cocoa powder, baking soda, and salt until fully combined.
In another large mixing bowl cream together the butter, granulated sugar, and brown sugar. Once fully mixed add in the egg and vanilla extract and mix until well combined.
Slowly add in the dry ingredients to the wet butter mixture and mix until just combined, scraping the sides of the bowl to ensure all ingredients are fully mixed.
Using a cookie scoop or small spoon evenly distribute the cookie dough into all cavities of the mini muffin pan.
Bake at 350°F for 12 -13 minutes.
Remove from the oven and using the back of a small measuring teaspoon, gently press in the center of each cookie cup.
Allow cookie cups to cool a few minutes longer in the pan and then transfer to a wire rack to finish cooling.