Preheat oven to 375°F. Spray a muffin tin with cooking spray (the recipe will make 9 muffins).
In a large bowl, whisk together the brown sugar, buttermilk, egg, vanilla, and vegetable oil until smooth.
In another bowl, whisk together the flour, baking soda, baking powder, salt, cinnamon, nutmeg, ginger, and allspice. Pour the dry ingredients into the wet ingredients, and stir gently with a spatula just until combined.
Add the grated carrots, raisins, and walnuts, and stir just until combined.
Divide the batter between 9 muffin cups. Bake for 17–19 minutes, or until light golden brown on top and a toothpick inserted into the center of a muffin comes out clean.
Let cool for 5 minutes in the muffin tin, then remove the muffins (run a butter knife around the edges if they’re stuck). Enjoy the muffins while warm or let cool completely.
Notes
This recipe makes lightly sweet muffins. For sweeter muffins, increase the brown sugar to ¾–1 cup. Don't have buttermilk? Make your own buttermilk by pouring 2 teaspoons of vinegar into a glass measuring cup. Fill the cup to the ½ cup line with milk (any variety of cow's milk will work, such as 1%, 2%, whole, etc.). Let sit for 5 minutes and use in the recipe in place of the buttermilk.