Preheat the oven to 350℉. Grease a 12-cavity muffin tin with nonstick cooking spray, line each cavity with a cupcake wrapper, then grease each wrapper to prevent the muffins from sticking.
Peel the bananas and add to a large mixing bowl. Thoroughly mash the bananas with a fork until they resemble applesauce, then add the melted butter, granulated sugar, dark brown sugar, egg, vanilla extract, sour cream, and buttermilk and stir to combine.
In the same bowl, sprinkle in the all-purpose flour, ground cinnamon, baking soda, baking powder, and salt and fold until just combined-- do NOT overmix! Fold in the walnuts and chocolate chips.
Use a teaspoon to neatly scoop the batter into the prepared muffin wrappers, filling each one to the top. Generously sprinkle the turbinado sugar overtop each muffin. Optionally, sprinkle a few more chocolate chips over each muffin, then bake for 18-20 minutes, or until a toothpick comes out with a few wet crumbs. Allow the muffins to cool in the pan for 2-3 minutes before turning out onto a wire cooling rack to finish cooling.
