Position a rack in the middle of an oven and preheat to 3250F. In a food processor fitted with the metal blade, combine the sugar and vanilla bean and process until fine; set aside.

In a bowl, using an electric mixer set on high speed, beat the egg yolks until very thick and pale, about 4 minutes. Place the vanilla sugar in a sifter and gradually sift directly onto the yolks, discarding any large pieces of vanilla bean. Continue to beat until the mixture is thick and pale and tripled in voluine1 about 5 minutes. Gradually beat in the orange juice and zest.

In a clean bowl, using clean, dry beaters, beat the egg whites and salt until stiff but not dry; set aside. Combine the cake meal and potato starch in the sifter and sift the mixture directly onto the egg yolk mixture. Using a rubber spatula, fold in gently but thoroughly. Then fold in the egg whites, again working gently but thoroughly.

Spoon into a tube pan 10 inches in diameter and 4 inches deep. Bake until golden and the top springs back when gently touched, about 50 minutes. Invert the cake in its pan onto the neck of a bottle. Let cool completely, about 1 hour.

To remove the cake from the pan, run a sharp knife around the edges of the pan to loosen the cake, then invert onto a cake plate. Carefully lift off the pan. Serve immediately, or cover with a cakE dome and store at cool room temperature for up to 8 hours.