Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners or grease it lightly.
In a large mixing bowl, whisk together the all-purpose flour, whole wheat flour, baking soda, salt, and sugar until well combined.
In a separate bowl, mix the buttermilk, egg, and melted butter until smooth.
Pour the wet ingredients into the dry ingredients and stir gently until just combined. If using, fold in the raisins or currants. Be careful not to overmix.
Divide the batter evenly among the muffin cups, filling each about ¾ full.
Bake in the preheated oven for 18-20 minutes, or until the muffins are golden brown and a toothpick inserted in the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.