Preheat the oven to 350ºF. Cream butter with orange zest and 1 cup of the sugar until light and fluffy.
Add the egg and vanilla and beat until combined. Meanwhile, toss the cranberries with 2 tablespoons of flour, then 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 the cranberries.
Grease a 9-inch square baking pan (or something similar) with butter or coat with non-stick spray. Spread batter into pan. Sprinkle batter with remaining tablespoon of sugar. Bake for 35 minutes, then check for doneness by touching the top gently or by inserting a toothpick. If necessary, return pan to oven, check every five minutes or so — it took my cake a little bit over 45 minutes to cook. (Note: Baking for as long as 50 minutes might be necessary, especially if you made the batter in advance.) Let cool at least 15 minutes before serving.
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