Preheat the oven to 400°F (200°C) and line a 12-cup muffin tin with paper liners. In a large bowl, whisk the melted butter and sugar until well blended, about 1 minute. Whisk in the egg, buttermilk, orange zest, and vanilla until smooth.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.

Pour the dry ingredients into the wet and fold gently with a rubber spatula just until no streaks of flour remain, about 10 to 12 strokes. Gently fold in the cranberries.

Divide the batter evenly among the muffin cups, filling each to the top. Bake for 5 minutes at 400°F, then reduce the oven temperature to 375°F (190°C) and continue baking for 13 to 16 more minutes, until the tops are golden and a toothpick inserted in the center comes out with a few moist crumbs. Cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.

Once the muffins are completely cool, whisk the powdered sugar with 2 tablespoons of orange juice until smooth, adding a little more juice if needed for a drizzling consistency. Drizzle the glaze over the muffins and, if you like, sprinkle with a pinch of orange zest. Serve once the glaze sets, about 10 minutes.