01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or lightly grease each cup.
02 - In a large bowl, whisk together all-purpose flour, whole wheat flour, sugar, baking soda, baking powder, and salt until well combined.
03 - In a separate bowl, whisk together buttermilk, melted butter, and egg until smooth.
04 - Pour wet ingredients into dry ingredients and stir gently with a spatula until just combined; do not overmix.
05 - Fold in golden raisins and orange zest, if using, until evenly distributed.
06 - Divide batter evenly among prepared muffin cups. Sprinkle tops with demerara or turbinado sugar if desired.
07 - Bake for 16-18 minutes, or until a toothpick inserted into the center comes out clean and tops are lightly golden.
08 - Let muffins cool in tin for 5 minutes, then transfer to a wire rack to cool completely.