Preheat oven to 400° F. Line a muffin tin with paper liners, or spray it with nonstick cooking spray.
Beat the buttermilk, bananas, oil and egg together in the bowl of a stand mixer, until thoroughly combined.
Mix together the flour, brown sugar, baking powder and salt in a separate bowl. Stir this into the wet mixture until the wet and dry just start to combine. Carefully fold in the chocolate chunks using as few strokes as necessary. (It's very important not to over-beat the batter, a few dry lumps are okay.)
Fill each muffin cup about 3/4 of the way. Sprinkle the tops with a pinch or two of decorating sugar.
Bake for 20 minutes, until they are starting to brown and a tester inserted in the center comes out clean. Turn the muffins out onto a cooling rack. Serve them while warm and while the chocolate is melty if you can!
Once cool, the muffins can be stored in a sealed container for up to three days. Makes about 16 muffins
* To brown bananas: store bananas in the refrigerator until they are really brown. Once brown use the bananas right away. You can also place them in a freezer-safe plastic bag (with peels on) and freeze the bananas for up to 2 months. To thaw, leave them in the fridge overnight or remove from the plastic bag, transfer to a microwave-safe plate and microwave them for about 30 seconds.