Preheat oven to 350 degrees.
Cream butter with orange zest and 3/4 cup of sugar until light and fluffy.
Add the egg and vanilla and beat until combined. Whisk together the remaining flour, baking powder and salt.
Add the flour mixture to the batter a little at a time, alternating with the buttermilk.
Fold in cranberries.
Grease a muffin tin or with butter or spray paper muffin cups with non stick spray. Spoon batter into cups 3/4 full.. Sprinkle muffins with remaining tablespoons of sugar.
Bake for 35 minutes. Check with a toothpick for doneness. If necessary, return pan to oven for a couple of more minutes.
(Note: Baking for as long as 10 minutes more might be necessary.)