Heat oven to 325F and coat a 12-cup muffin tin with avocado nonstick spray.
In a small mixing bowl, whisk together the buttermilk, oil, honey, egg, and zest. In a large mixing bowl, whisk together the bran, flour, baking powder, baking soda, and salt.
Pour the wet mixture into the dry mixture and mix until just combined. Gently fold in the berries. Working quickly, divide the batter between the 12 muffin cups. Sprinkle each muffin top with coconut sugar.
Bake muffins for about 20 minutes, rotating pan halfway through baking for even browning, until a toothpick inserted into the center of muffins comes out with just a few crumbs attached. Let muffins cool in the pan on a wire rack for 10 minutes before removing from tin.
