Heat the oven to 425°F (220°C), and line a 12-cup muffin pan with paper liners or grease each cup well. Zest the orange before squeezing it for juice, and set the melted butter aside to cool slightly.

Whisk the flour, baking powder, baking soda, and fine salt together in a medium bowl. This distributes the leaveners evenly so the muffins rise consistently.

In a separate large bowl, whisk together the melted butter, sugar, eggs, vanilla, orange zest, orange juice, and buttermilk until combined. The mixture may look slightly loose, which is fine before the dry ingredients are added.

Add the dry mixture to the wet mixture and stir just until no dry flour remains. Keep the batter somewhat thick and lumpy, since overmixing can make the finished muffins dense.

Gently fold in the cranberries, using care not to crush them. If using frozen cranberries, add them straight from the freezer; tossing them with a tablespoon of the measured flour can help keep them suspended in the batter.

Divide the batter among the prepared muffin cups, filling each about three-quarters full. Sprinkle the tops with 2–3 tablespoons of coarse or sparkling sugar if you want a crisp, lightly glittering finish.

Bake for 5 minutes at 425°F (220°C), then reduce the oven temperature to 350°F (175°C) without opening the door. Continue baking for 15–20 minutes, until the tops are lightly golden and a toothpick inserted in the center comes out clean or with a few moist crumbs.

Cool the muffins in the pan for about 5 minutes, then transfer them to a wire rack. Serve warm or let them cool completely before storing.