Preheat your oven to 350°F (175°C). Prepare your muffin tin by lining it with paper liners or greasing it with non-stick spray to prevent sticking.
In a large mixing bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking powder, baking soda, and salt until everything is combined and there are no lumps.
In a separate mixing bowl, combine the granulated sugar, packed brown sugar, eggs, buttermilk, vegetable oil, and vanilla extract. Whisk thoroughly until the mixture is smooth and the sugars have dissolved completely.
Gradually add the dry ingredients to the wet mixture, stirring gently until just combined. Make sure not to overmix; it’s perfectly fine if a few small lumps remain in the batter.
Gently fold in the chocolate chips and chopped nuts (if you’re using them) until they are evenly distributed throughout the batter.
Using a spoon or ice cream scoop, fill each muffin cavity about 3/4 full with the batter to allow room for rising.
Place the muffin tin in the preheated oven and bake for 18-20 minutes. Muffins are done when a toothpick inserted into the center comes out clean or with just a few moist crumbs attached.
Once baked, remove the muffin tin from the oven and let the muffins cool in the pan for about 5 minutes. Then, transfer them to a wire rack to cool completely.
Notes
Serve warm for the best flavor. Consider dusting with powdered sugar or drizzling melted chocolate for presentation.