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