Mix Dry Ingredients: In a bowl, combine the flour, cocoa powder, baking soda, baking powder, salt, and brown sugar. Set aside.
Mix Wet Ingredients: In a measuring cup or another bowl, whisk together the eggs, oil, sour cream, milk, and vanilla extract.
Combine Mixtures: Pour the wet ingredients into the bowl with the dry ingredients. Stir until just combined. Add the chopped chocolate or chocolate chips and mix in.
Optional Resting: Cover the batter and let it sit for 20 minutes for extra fluffy muffins.
Fill Liners: Spoon the batter into cupcake liners and sprinkle more chocolate on top.
Bake: Bake at 425°F for 5 minutes, then lower the temperature to 350°F and bake for an additional 8 minutes, or until the muffins spring back when touched.