Have all ingredients at room temperature, 68┬░ to 70┬░F. Preheat the oven to 350┬░F. Have ready 1 ungreased 10-inch tube pan.
Sift together 3 times:
2/3 cup matzo meal
1/3 cup potato starch
1/4 teaspoon salt
In a medium bowl, beat on high speed until thick and pale yellow, about 2 minutes:
9 large egg yolks
1 cup sugar
Beat in:
1 teaspoon grated orange zest
1 teaspoon grated lemon zest
1/4 cup fresh orange juice
1 tablespoon fresh lemon juice
Gradually add the dry ingredients, beating on low speed just until smooth. Scrape the batter into a large bowl and set aside. In another large bowl, beat on medium speed until soft peaks form:
9 large egg whites
1/2 teaspoon cream of tartar
Gradually add, beating on high speed:
1/2 cup sugar
Beat until the peaks are stiff but not dry. Use a rubber spatula to fold one-quarter of the egg whites into the egg yolk mixture, then fold in the remaining whites. Spoon the batter gently into the pan and spread evenly. Bake until the top springs back when lightly pressed and a toothpick inserted into the center comes out clean, 50 to 55 minutes. Let cool upside down for at least 1 1/2 hours, setting the tube over a bottleneck or resting the pan on 4 glasses. To unmold, see To Unmold Sponge, Chiffon, and Angel Cakes.
