Heat oven to 375ºF. Coat an 8-inch-by-4-inch loaf pan with cooking spray.
Grate zest from 1/2 of the orange into a large bowl. Cut the orange in half and squeeze in all of the juice. Whisk in eggs, buttermilk, oil, sugar, brown sugar, and salt until blended. Scatter baking powder over the top, and let stand until foamy, about 3 minutes.
Stir together cornmeal, flour, and cranberries, and add them to the wet ingredients, stirring just until batter is almost free of lumps.
Spoon into prepared pan and bake until a toothpick inserted in the center comes out clean, 35 to 40 minutes. Cool on a rack.
Tip: If you don’t have fresh cranberries, use blueberries or raspberries instead.