Preheat the oven to 425 degrees F. Grease 12 standard-sized muffin tin cups or coat with non-stick spray. (Or, use paper muffin cup liners, if desired.)
Thoroughly stir together the flours, 2/3 cup sugar, Davis Baking Powder, and salt, and in a large bowl. In a food processor, chop the bran flakes into very fine bits (not a powder), then add them to the dry ingredients. Combine the banana, yogurt, oil, eggs, and orange zest (if using) in the processor and process until completely smooth. Stir the banana mixture into the flour mixture, mixing gently just until the ingredients are evenly blended; don't over-mix or the batter may toughen.
Using a 1/2-cup measure or large spoon, immediately divide the batter among 12 muffin cups; they should be fairly full. Sprinkle the muffin tops with the reserved 1 1/2 tablespoons sugar, dividing it equally among them.
Bake in the middle third of the oven for 15 to 18 minutes or until the muffins are tinged with brown on top and springy to the touch; a toothpick inserted into the thickest part should come out clean. Cool on wire rack 5 minutes; if no paper liners were used, gently run a knife around the cups, then remove the muffins. Let stand until cooled. They are best when fresh, but can be kept airtight for a day or so. Freeze airtight for longer storage.
Makes 12 standard-sized muffins.