Preheat oven to 375F / 190C. Lightly grease a muffin tin or line with paper cases.
Stir the wheat bran into the buttermilk and set aside for 10 minutes to soften the bran.
In a large bowl, beat together the banana, oil, egg, sugar and vanilla, then stir in the buttermilk mixture.
Sift in both flours, bicarb of soda, baking powder, cinnamon and salt, and gently stir, taking care not to over mix. Gently fold in the raisins.
Spoon the mixture into the prepared muffin tin and bake for 15 minutes, or until an inserted skewer comes out clean.
Allow to cool in the tin for 5 minutes, then transfer the muffins to a wire rack to cool completely.
VariationsTurn these into vegetable muffins by adding grated carrot or parsnipMake them into Carrot Cake Bran Muffins by adding 1 cup grated carrot, an extra ½ tsp ground cinnamon and ½ tsp ground nutmeg.For a tropical taste, add ½ cup chopped pineapple and ¼ cup shredded coconut.Storage: Store the muffins for up to 4 days in an airtight container lined with paper towel. If placing the muffins in two layers, place paper towel between the layers (this stops the muffins getting soggy).Freezing: These muffins are freezable. Just allow them to cool completely then freeze in an airtight bag or container.