If you don't have buttermilk on hand, make your own by pouring 2 generous teaspoons of lemon juice or white vinegar into a measuring cup, and then adding milk up to the 3/4 cup line. Allow to sit at room temperature for about 10 minutes or until it begins to thicken and curdle.
Two large oranges gave me enough juice and zest for this recipe.
If making this recipe in two 8 1/2 x 4 1/2 x 2 1/2-inch loaf pans or one Bundt pan, start checking for doneness at 45 minutes. Cake is done when a toothpick inserted in center comes out clean (after probably no longer than an hour).
Adapted from Ina Garten recipe on FoodNetwork.com.