First, preheat your oven to 350°F (175°C). While the oven heats up, line a standard 12-cup muffin tin with paper cupcake liners. This step prevents the cupcakes from sticking and makes cleanup much easier.
In a large mixing bowl, sift together the all-purpose flour, unsweetened cocoa powder, granulated sugar, baking powder, and salt. Sifting is an important step as it removes any lumps and aerates the ingredients, leading to a lighter cupcake.
In a separate medium-sized bowl, whisk together the large eggs until they are light and frothy. Then, add the buttermilk and vegetable oil, continuing to whisk until the mixture is smooth and well combined. This wet mixture is key to the cupcakes' moisture.
Gently pour the wet ingredients into the bowl of dry ingredients. Using a spatula or wooden spoon, stir until everything is just combined. Be careful not to overmix the batter, as this can lead to tough, dense cupcakes.
Now it’s time to add the dark chocolate chips. Gently fold them into the batter until they are evenly distributed. This ensures you get a delightful bit of melted chocolate in every single bite of your finished cupcake.
Carefully fill each cupcake liner about two-thirds full with the batter. This gives them enough space to rise without overflowing. Bake in the preheated oven for 18-20 minutes. You can check for doneness by inserting a toothpick into the center of a cupcake; it should come out clean.
Once baked, remove the cupcakes from the oven and let them cool in the muffin tin for a few minutes before transferring them to a wire rack to cool completely. While they cool, prepare your blackberry buttercream frosting. Once the cupcakes are fully cooled, pipe or spread the frosting on top and garnish with a fresh blackberry.