Slowly add the egg, orange zest, and vanilla extract, mixing on low speed until smooth. Whisk together the flour, baking powder, and salt in a separate mixing bowl.
Add the dry ingredients to the wet ingredients and mix on low speed until almost combined. Finally, add the buttermilk and mix on low speed until the ingredients are combined. Carefully fold in the cranberries by hand.
Spray a 9" by 9" baking dish with non-stick cooking spray, or lightly grease it with melted butter. Transfer the batter to the prepared baking dish and use an offset spatula to spread evenly.
Sprinkle about 2 tablespoons of coarse sugar over the top just before baking. Bake in a preheated 350-degree oven for 45 minutes or until a toothpick inserted into the center comes out clean.