½ stick (4 tablespoons) unsalted butter
¾ cup packed dark brown sugar
Juice of 1 medium navel orange
Zest of 3 medium navel oranges
2 blood oranges, peeled and cut into thin slices
5-7 kumquats, sliced (and deseeded, if necessary)
½ cup granulated sugar
2 large eggs
½ cup extra-virgin olive oil (or a mixture of extra-virgin and light, if you prefer)
1½ cups unbleached all-purpose flour
1 teaspoon baking powder
½ teaspoon baking soda
½ teaspoon salt (fine sea salt or regular table salt are equally acceptable)
4 teaspoons fine cornmeal
1 teaspoon rum (or vanilla, or orange blossom water)
4-5 dashes orange bitters (optional)
It’s best to prep your oranges and kumquats first. I find that using a Microplane is the easiest way to zest citrus fruit, but a regular zester or the fine side of a box grater can also work. The zest will be incorporated into the cake batter as a flavor enhancer, so the end result, whichever method you use, needs to be as fine as possible and free of pith (the white, slightly spongy part under the peel, which will impart a bitter flavor). When preparing the blood oranges, be sure to remove as much of the excess pith as possible before slicing. A serrated knife is helpful for ensuring thin slices, but any type of sharp knife will work. Once your oranges are sliced, lay the slices on a paper towel to soak up some of the extra juice. Note: If you can’t find blood oranges at the store or if you prefer a slightly sweeter taste (blood oranges are intense), you can use regular navels or cara cara navels, which have a lovely pink hue, instead. The end result won’t be quite as striking, but it will still taste delicious.
Preheat the oven to 350 degrees Fahrenheit, then butter a 9-inch round cake pan. Set this aside for the moment.
Melt the butter in a saucepan over medium heat, then add the brown sugar and stir until it’s dissolved — this will take several minutes. It’s OK if the butter-sugar mixture looks slightly grainy, but if it’s too grainy, then the cake might not turn out of the pan correctly. On the other hand, you don’t want to let the butter and sugar boil and become a proper caramel, either. Once the sugar is fully dissolved into the butter, pour it all into the buttered cake pan and arrange the blood orange and kumquat slices on top (I like to fill gaps between the orange slices with the smaller kumquats).
Whisk the sugar, oil, eggs, orange juice, zest, flavoring and bitters together in a small bowl. Then, in a large bowl, whisk the flour with the cornmeal, baking powder, baking soda and salt. Pour the oil and sugar mixture into the flour mixture and stir until just combined.
Carefully spread the batter over the oranges and bake for 30-35 minutes, or until a cake tester or toothpick inserted into the center comes out clean. The cake should be thoroughly done, but not over-baked. Cool the cake in the pan for about 15 minutes, and then carefully invert it onto a serving plate. If some oranges stick in the bottom of the pan, just peel them out and pat them back into place on top. If some of the brown sugar mixture remains in the pan, you can either scrape this out and spoon it over the cooling cake, or you can scrape it out and eat it; depending on how much is left, I recommend the latter course of action. Serve with unsweetened or lightly sweetened whipped cream. The cake can be made a day in advance of your meal, and, in fact, often tastes better the second day as the flavors have had a chance to develop.