These little cakes are enriched with whole grains and sweet carrots and raisins, but have a lighter texture than the bran muffins you may have tried in the past.
For the batter:
Preheat your oven to 400° and set a rack to the middle position. Line the cups of a standard 12-cup muffin pan with paper liners or grease them lightly with butter.
Using a standing or handheld mixer, cream the butter and brown sugar in a large bowl until light and fluffy. Beat the eggs into the butter mixture one at a time, beating well after each addition. Add the vanilla and stir to combine.
In a separate large bowl, whisk together the all-purpose flour, bran, whole-wheat flour, baking powder, cinnamon, and salt. Add one-third of the flour mixture to the butter mixture and beat until smooth. Add half of the buttermilk, mixing gently until combined.
Add another third of the flour mixture and beat until smooth. Add the remaining half of the buttermilk and mix gently. Finish by adding the remaining third of the flour mixture. Then stir in the carrots, raisins, and walnuts until just mixed through. Divide the batter evenly among the muffin cups.
For the Nutty Topping (optional):
Stir the finely chopped walnuts together with the poppy and sesame seeds. Sprinkle the topping over the batter in each cup.
Bake until muffins are set and lightly golden at the edges, 25 to 30 minutes. Let them cool slightly before unmolding.