Using an electric mixer fitted with the whisk attachment or a handheld mixer, whisk eggs and sugar on high speed until thick and pale, about 4 minutes. Reduce speed to low and slowly add in chocolate mixture. Stop the mixer. Add flour mixture and fold, using a rubber spatula, just until combined. Do not overmix.
Divide batter evenly using a silicon muffin pan or muffin tin with muffin liners, filling them about ¾ full. Sprinkle extra chopped chocolate on top prior to baking, if desired. Bake for 13-15 minutes or until a toothpick inserted into the center of the muffins comes out with moist crumbs and not dry. Be careful not to overbake as this will make the brownies dry. Transfer to a wire rack and let cool for 10 minutes before removing from pan. Allow muffins to cool completely on a wire rack.
Note: I served these with a delicious homemade chocolate ganache.