Preheat the oven to 400°F (200°C) and spray a muffin tray with non-stick spray, or line it with muffin liners.
Sift all purpose flour and cocoa powder into a mixing bowl and add baking soda, salt and instant espresso powder. Mix together.
Prepare your homemade buttermilk by adding 1 tablespoon lemon juice (or distilled white vinegar) to a measuring jug. Add room temperature milk up to the 1 cup (240ml) line. Leave it for a few minutes to curdle.
Add butter and dark chocolate (broken up or chopped) to a microwave safe bowl. Microwave in 15 second intervals bringing it out to stir every 15-seconds until melted.
Add light brown sugar to the melted butter and chocolate and mix it in.
Add the buttermilk, eggs, oil and vanilla extract and whisk in until well mixed.
Add the wet ingredients to the dry and mix into a thick batter. Be careful not to overmix.
Add chocolate chips and fold them into the batter. Divide the batter evenly between the partitions of your muffin tray (sprayed with non-stick spray).
Add a few more chocolate chips directly to the tops of each muffin before baking.
Bake at 400°F (200°C) for 5 minutes and then turn the temperature down to 350°F (180°C) and bake for a final 15-17 minutes or until a toothpick comes out with moist crumbs or melted chocolate but NO wet batter.
Let the muffins cool in the pan for a few minutes and then transfer them to a wire cooling rack to cool completely. The muffins are quite fragile when still warm, because they are deliciously soft. So handle them with care.