Prepare the Dry Ingredients. In a large mixing bowl, whisk together all-purpose flour, cocoa powder, granulated sugar, baking powder, and a pinch of kosher salt until well combined. Set aside.
Mix the Wet Ingredients. Whisk together the eggs, sour cream, vegetable oil, buttermilk, and vanilla extract in a separate bowl until smooth and fully incorporated.
Combine the Batter. Gradually pour the wet ingredients into the dry ingredients, mixing gently until combined. Take care not to overmix, as it can impact the texture of the muffins. Gently fold the chocolate chips, making sure they are evenly distributed throughout the batter.
Let the Batter Rest (optional, but recommended). Cover the batter and let it sit at room temperature for 20 minutes. This step hydrates the flour and helps create tall, bakery-style muffin tops.
Preheat the Oven. While the batter rests, preheat the oven to 425°F and line a 12-cup muffin pan with paper liners.
Fill the Muffin Cups. Using a cookie scoop or spoon, divide the batter evenly among the muffin liners, filling each ¾ full. Sprinkle additional chocolate chips on top for extra decadence.
Bake in Two Stages. Bake the muffins at 425°F for 5 minutes to encourage a high rise. Without opening the oven, reduce the temperature to 350°F and continue baking for 8–10 minutes, or until the tops are set and a toothpick inserted in the center comes out with a few moist crumbs. Avoid overbaking to maintain a soft texture.
Cool and Serve. Remove the muffins from the oven and, if desired, sprinkle lightly with flaky sea salt for contrast. Let them cool in the pan for 5 minutes, then transfer to a wire rack to cool completely. Enjoy!