Preheat oven to 425F. Line or grease 20 standard-sized muffin tin cups. Set aside.
In a large bowl whisk eggs, oil, buttermilk, sour cream, and vanilla together until combined.
Add flour, sugar, cocoa, baking powder, salt, and 3/4 cup chocolate chips; stir until just combined.
Fill the muffin batter into the prepared liners until almost full.
Sprinkle remaining 2 tablespoons of chocolate chips over the batter.
Bake at 425F for 5 minutes.
Without removing the muffins, lower the temperature to 350F and continue baking additional 13-15 minutes, or until a toothpick inserted into the center comes out clean.
Let cool in pan for 5 minutes.
Remove from the muffin tin and let cool to room temperature on a wire cooling rack.
Video
Notes
Buttermilk: I recommend using full-fat buttermilk. It makes for softer, moister muffins. However, low-fat buttermilk will work.
Sour cream: I recommend using full-fat sour cream for moister muffins. Reduced-fat sour cream is ok, but I don't recommend using fat-free sour cream.
Flour: It's important to properly measure the flour and cocoa powder to avoid dry, heavy muffins. Sift or stir the flour/cocoa powder, lightly spoon into the measuring cup, and level.