Preheat the oven to 350°F. Line two medium-sized muffin tins with 14 paper cupcake liners, spacing them evenly between two pans.
In a large mixing bowl, combine the flours, baking powder, baking soda, and salt. In a smaller mixing bowl, whisk the eggs lightly. To the eggs, add the oil, Stevia In The Raw®, Sugar In The Raw® Organic White, yogurt, buttermilk, and vanilla extract, whisking until they are well combined. Pour the liquid ingredients into the dry, add in 4 mashed bananas, and whisk just until blended. Fold in the walnuts and half of the chopped chocolate. Spoon the batter into the prepared muffin tins, filling cups almost to the top. Top each muffin with some chocolate chunks and a few banana slices. Sprinkle the tops of the muffins with the remaining combined Stevia In The Raw® and Sugar In The Raw® Organic White.
Bake for 25 to 30 minutes, or until the muffins are deep golden brown and spring back lightly when pressed gently in the center. Immediately turn the muffins out on to a wire rack to cool.