Center the oven rack; preheat the oven to 400℉
Bring a large pot of salted water to a boil. Add the oranges to the boiling water, cover the pot, and let the oranges boil until perfectly soft - 60-90 minutes.* Drain, cool, and cut the fruit into quarters. *Note: The longer the oranges boil, the less bitter it will be. Put the quartered oranges in the bowl of a food processor or blender. Process to a fairly fine puree.
In a large bowl, whisk the eggs until pale and thick. One ingredient at a time, whisk in the almond flour, sugar, pinch of salt, baking powder, and the orange puree.
Pour the batter into the prepared pan, and bake until firm, about 60 minutes. *Note: Check the cake after 40 minutes. If the batter is turning too dark, cover loosely with aluminum foil, then continue baking). (I have baked for 45 minutes only, for a super moist cake and according to my own oven. Baking times are arbitrary, and you should go by look and feel as a guide as well.) The cake should be springy to the touch.
When done, tranfer the pan to a wire rack to cool. Run a plastic knife or thin spatula between the pan and cake, then release the cake onto a serving platter or cake stand. Dust with the optional confectioners' sugar.
