Preheat the oven to 350 degrees F. Grease a 12-cup muffin tin and set aside (or line with paper cups)
In a small bowl, combine the bran and buttermilk and set aside. In the bowl of a standing mixer fitted with the paddle attachment or a large mixing bowl, cream the butter and sugar for about 3 minutes, until light and fluffy. Mix in the Greek yogurt and cream 2 additional minutes.
With the mixer on low, add the eggs, one at a time. 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 all purpose flour, whole wheat flour, baking powder, baking soda, and salt. With the mixer on low speed, slowly add the flour mixture to the batter, just until combined. With a rubber spatula, fold in the bananas, walnuts, and raisins.
Fill the muffin cups to the top and bake for 25 to 30 minutes, until a toothpick comes out clean.
Notes
*Ingredient Note: If you don't have buttermilk, simply combine 1 cup regular milk and 1 tablespoon white vinegar or lemon juice. Stir, set aside 5 minutes, then use as directed. Store muffins for up to four days, following my How to Store Muffins and Other Quick Breads or tightly wrap individually in two layers of plastic and freeze for up to two months.