Preheat oven to 375 F. Line 12 muffin cups with liners, or spray with baking spray.

In a large bowl, combine flours, sugar, baking powder, xanthan gum, salt, and baking soda. Set aside.

In a separate bowl, whisk together milk, yogurt, vegetable oil, eggs, vanilla extract, and zest, if using.

Add wet ingredients mixture all at once to the bowl of dry ingredients, stir just until combined, then fold in the cranberries and walnuts.

Divide batter between 12 prepared muffin cups, filling each to almost full. Sprinkle with coarse sugar, if using.

Bake in a 375 degree F oven for 25 to 28 minutes or until golden and a wooden toothpick inserted in centers comes out clean.

Cool in muffin cups on a wire rack for 5 minutes.

Remove from muffin cups; serve warm.