Preheat oven to 375°F (190°C).
Combine wheat bran and buttermilk; let stand. Mix together egg, oil, banana and vanilla and add to bran mixture.
Combine flour, baking powder, baking soda, salt, cocoa and sugar and stir well. Add dry ingredients to bran mixture and stir just until blended. Spoon into prepared muffin cups. Bake 20 – 25 minutes.
* To substitute for 3/4 cup (175 mL) buttermilk, use 1 tbsp. (15 mL) lemon juice plus enough milk to make 3/4 cup (175 mL).