Preheat oven to 350F. Spray 9x5" loaf pan lightly with cooking spray and set aside. In a large bowl, combine flour, wheat bran, sugar, ground flax, baking soda, baking powder, cinnamon, and salt. In a medium bowl, whisk together bananas, yogurt, eggs, butter, and vanilla. Add wet ingredients to dry ingredients and stir just until moistened. Fold in chocolate, don't over mix batter. Pour into pan. Bake for 50 min. Cool on rack.