Preheat oven to 400F degrees. Prepare muffin pan by greasing cups and top surface or lining with paper cups.
In one bowl, mash bananas with fork. You should have about 1 cup.
Add pure vanilla extract, yogurt (or sour cream), and oil. Stir until combined and set aside.
In second bowl, sift flour, baking soda, and salt. Set aside.
In mixer bowl, add egg and sugar. Mix until light, creamy, and fluffy, a few minutes.
Decrease speed and slowly spoon in banana mixture. Mix until combined.
Add dry ingredients slowly until just combined. Then pour in buttermilk and mix just enough to combine.
Add chocolate chips and chopped chocolate and mix just to distribute. Do not over-mix.
Divide batter in pan and fill to the top. Yes, to the top. Sprinkle sugar on top.
Bake at 400F for 5 minutes then lower temperature to 350F and continue baking for another 20 minutes or until toothpick inserted in center comes out clean.
Remove from oven and let cool for a few minutes. Then remove each muffin and place on wire rack to cool.