Preheat your oven to 375 degrees Fahrenheit and line your muffin tin with paper liners or coat lightly with cooking spray.

In a large bowl, whisk together the wheat bran, all-purpose flour, whole wheat flour, baking powder, baking soda, salt, cinnamon, and nutmeg.

In a separate medium bowl, whisk together the melted butter, molasses, eggs, buttermilk, and vanilla extract until smooth.

Pour the wet mixture into the dry ingredients and fold gently with a wooden spoon or spatula until just combined.

Gently fold in the raisins and walnuts if using.

Spoon the batter evenly into each of the 12 prepared muffin cups, filling each about two-thirds full.

Transfer the muffin tin to the preheated oven and bake for 20 to 22 minutes.

Let the muffins cool in the pan for 5 minutes, then turn them out onto a wire cooling rack.