In a large mixing bowl sift together the flour, cocoa powder, brown sugar, baking powder, baking soda, and salt. Mix using a spatula and set aside.
In a large bowl whisk together the eggs, buttermilk (see recipe notes), oil, black coffee, and vanilla essence using a hand mixer or wire whisk for 10 minutes.
With a rubber spatula fold the wet ingredients into the dry ingredients and mix only until the ingredients are combined. (Do not over mix the batter or the muffins will be hard)
Preheat the oven at 190-degree Celcius and line muffin tray with paper liners. Spoon in an equal amount of batter into muffin cups, filling only up to ⅓ in each muffin cup.
Sprinkle chocolate chips or dark chocolate on top of each muffin. Place in the oven and bake at 190-degree for about 18 -23 minutes (mine took exact 20 minutes).
Transfer baked muffins to a wire rack and let it cool completely before storing or serving.