Preheat the oven to 350 degrees F. Place paper liners into 1 (10 or 12-cup) muffin tin.
Combine the bran and buttermilk and set aside. Cream the butter and sugar in the bowl of an electric mixer fitted with the paddle attachment for about 5 minutes, until light and fluffy. With the mixer on low, add the eggs, 1 at a time. Scrape the bowl and then add the molasses, orange zest, and vanilla. (The mixture will look curdled.) Add the bran/buttermilk mixture and combine.
In a separate bowl, sift together the flour, baking powder, baking soda, and salt. With the mixer on low speed, slowly add the flour mixture to the batter just until combined. Don't overmix it! Fold in the raisins, bananas and walnuts with a rubber spatula.
With an ice cream scoop or large spoon, fill the muffin cups to the top and bake for 25 to 30 minutes, until a cake tester comes out clean.