Preheat oven to 350ºF. Spray cavities with vegetable spray or line with baking cups.
In large bowl, combine flour, sugar, baking powder and salt. In small bowl, combine egg, milk, melted butter, cranberries and orange zest; whisk until blended. Stir wet ingredients into dry ingredients, mixing only until moistened. Do not overmix. Divide batter into muffin cups. Sprinkle with remaining sugar.