Passover Hazelnut Sponge Cake

In a bowl, combine the egg yolks, 1/2 cup of the sugar, and both the zests and juices. Beat with an electric mixer until the mixture is thick and pale and holds a 3-second slowly dissolving ribbon when the beaters are lifted. In a second bowl using clean, dry beaters, beat the egg whites until foamy. Gradually beat in the remaining 1/2 cup sugar, and continue to beat until stiff peaks form. Gently fold the whites into the egg mixture, then fold in the hazelnuts, the matzoh meal, potato starch, salt, and vanilla. Pour the batter into a ungreased 10-inch tube pan and smooth the top. Place the pan in a cold oven. Turn on the oven to 325 degrees F and bake until a toothpick inserted into the center of the cake comes out clean, about 45 minutes. Invert the cake still in the pan onto a wire rack. Let cool completely, then lift off the pan and transfer the cake to a serving platter.