Preheat the oven to 350°F Rub muffin tins with a 1/3-cup capacity with butter (or line tins with paper liners.) To encourage even baking, the author suggests filing alternate cups in a 24-cup tin, or to use two 12-cup tins. I never am good at following directions.
For the streusel topping, measure the flour, oat bran, sugars, and salt into a mixing bowl. Add the butter to the dry mixture. Rub the butter between your fingers, (or use a pastry blender) breaking it into smaller bits. Continue rubbing until the mixture feels coarse, like cornmeal. The more quickly you do this, the more the butter will stay solid, which is important for the success of the recipe.
Sift the dry ingredients into a large bowl, pouring back into the bowl any bits of grain that may remain in the sifter. Stir the carrots into the dry ingredients.
In a small bowl, whisk the buttermilk slowly into the melted butter, add the egg and whisk until thoroughly combined. Using a spatula, mix the wet ingredients into the dry ingredients and stir to combine.
Scoop the batter into 8 muffin cups. The batter should be slightly mounded above the edge. Sprinkle the streusel topping evenly over the mounds of batter and press it into the batter slightly.
Bake for 32 to 35 minutes, rotating the pans halfway through. The muffins are done when they smell nutty and the bottoms are dark golden-brown and the tops are crisp. Remove the tins from the oven, twist each muffin out, and place it on its side in the cup to cool. This insures that the muffin stays crusty instead of getting soggy. Eat warm from the oven or later that day, or freeze and reheat.