finely grated zest of 1 large blood orange and 1 large lemon (preferably meyer)
1 vanilla bean, split lengthwise and scraped
3 large eggs
1½ cups (12 ounces, 340 g) whole-milk ricotta
8 tablespoons (113 g) unsalted butter, melted
½ cup (80 g) sweet white rice flour
½ cup (75 g) corn flour (i.e. very finely ground cornmeal, NOT cornstarch)
½ cup (50 g) oat flour
2 teaspoons baking powder
½ teaspoon fine sea salt
1 cup (120 ml) heavy whipping cream
½ cup (4 ounces, 113 g) mascarpone
1 tablespoon organic granulated cane sugar
seeds from ½ vanilla bean (or ½ teaspoon vanilla extract)
4 large blood oranges
Position a rack in the center of the oven and preheat to 350ºF. Butter an 8-inch cake pan and line the bottom with a round of parchment paper.
Place the sugar in a large bowl and add the citrus zest and vanilla pod and scrapings. Rub the zest and vanilla seeds into the sugar to moisten and distribute. Remove the vanilla pod and reserve for another use. Beat the eggs into the sugar mixture, then the ricotta, and finally the butter. Place a strainer over the bowl and add the sweet rice, oat and corn flours with the baking powder and salt. Sift in and stir until well-combined.
Scrape the batter into the prepared pan, smooth the top, and bake until golden on top and a tester comes out clean, 50-60 minutes. Let cool until warm, then invert the cake onto a plate, peel away the parchment, and flip right side-up. Let cool completely unless serving warm.
To prepare the topping, whip together the cream, mascarpone, sugar and vanilla seeds or extract until the mixture just holds firm peaks. (If you overwhip and the mixture starts to look grainy, you can rescue it by folding in a few tablespoons of cream until it loosens up.) Chill until needed.
Prepare the oranges by cutting off ½-inch from each end. Place a cut side down and use a sharp knife (I like a small serrated knife) to pare away the peel and white pith, following the curve of the fruit. Turn the orange on its side and slice into rounds ⅜-inch thick. Repeat with the remaining oranges.
Dollop and spread the whipped mascarpone all over the top of the cake leaving a 1-inch border. Drain the orange rounds and place atop the cake in overlapping circles. Serve the cake right away, using a large serrated knife to cut slices (they will be messy – go with it). Extra cake will keep refrigerated airtight for up to a day or two.