(You will need a 10” angel food cake/tube pan with removable bottom. Do not grease the pan; grease prevents the egg white stabilized cake batter to climb up the walls of the pan.)
Remove the eggs from the refrigerator and let sit at room temperature for an hour.
Adjust the oven racks to accommodate your cake pan. It’s best to bake the cake on the middle rack of your oven.
Preheat your oven to 325 degrees F.
12 large eggs, separated
1½ cups sugar
grated zest of 1 lemon
grated zest of 1 orange
½ cup freshly squeezed orange juice
½ cup finely chopped walnuts (these are optional if you prefer a nut-free cake)
¼ cup potato starch
1 cup matzoh cake flour
pinch of salt
1 teaspoon of vanilla
Separate the eggs and measure out the remaining ingredients. Place the egg yolks in the bowl of a standing mixer fitted with the whisk attachment. Beat the yolks until frothy and lemon colored. Gradually add the sugar, a few tablespoons at a time. Add the lemon and orange zests. The yolks should be very thick. Transfer the yolk mixture to a large mixing bowl. Drizzle in the orange juice and the vanilla extract. Gently fold in the nuts, if you are using them.
Thoroughly wash the bowl and the whisk attachment from the standing mixer.
Sift together the potato starch and the matzoh cake flour. Fold these into the egg yolks a little at a time, being careful not to overbeat the batter. Place the egg whites in the clean bowl from the standing mixer fitted with the clean whisk attachment. (It’s important that both the bowl and the whisk are grease-free and dry.)
Beat the egg whites until frothy. Add a pinch of salt and beat until stiff and shiny but not dry, being careful not to overbeat. (It’s a delicate dance- the whites should form glossy peaks.) Fold the whites into the yolk/flour mixture, being careful not to deflate the mixture.
Gently pour the batter into the ungreased 10” angel food cake pan. Bake the cake on the middle rack of the pre-heated oven for 1 hour. Remove from the oven, place a cooling rack on top of the cake pan and carefully/slowly invert the pan on the cooling rack. Let the cake cool completely before removing the cake from the pan. It is easiest to slice this cake using a cake divider or a serrated knife. Cover and wrap any leftovers.