Preheat oven to 375ºF (190ºC). Position rack in center of oven. Butter or spray with a non stick cooking spray 12 (2¾ x 1½”) muffin cups. Set aside. In a large measuring cup or bowl whisk together the egg, buttermilk, oil, and vanilla extract. In another large bowl combine the flour, sugar, baking powder, baking soda, salt, and orange zest. Gently fold in the berries. With a rubber spatula fold the wet ingredients into the dry ingredients and stir only until the ingredients are combined. Do not over mix the batter or tough muffins will result. Fill each muffin cup almost full of batter, using two spoons or an ice cream scoop. Place in the oven and bake until a toothpick inserted in the center of a muffin comes out clean, about 20 minutes. Transfer to a wire rack and let cool for about 5 minutes before removing from pan. Dust with powdered sugar. Makes 12 regular sized muffins or 6 jumbo muffins. Note: If using frozen berries you may have to bake the muffins a little longer than the stated time. Also, if making jumbo sized muffins, again, you may have to bake the muffins a little longer.