Lemon poppyseed passover sponge cake

5 egg yolks
1-1/2 cups sugar
grated zest of 2 lemons (about 1 tablespoon)
1/2 cup fresh lemon juice
1/2 cup poppy seeds
1 cup matzo meal cake flour
3 tablespoons potato starch
10 egg whites
1 teaspoon cream of tartar
1/8 teaspoon salt
1 cup confectioners` sugar
1 (10-inch) angel food cake (tube) pan

Preheat oven to 325 degrees. Combine the egg yolks and 1 cup sugar in the bowl of an electric mixer and beat until thick, mousse-like and pale yellow, about 8 minutes. (Start the mixer on low speed and gradually increase it to medium, then high. When the mixture is the proper consistency, it will fall from a raised whisk in a thick ribbon.) Beat in the lemon zest, lemon juice and poppy seeds. Sift the cake flour and potato starch over the yolk mixture and gently fold to mix. Place the egg whites in a clean mixer bowl. Beat at low speed until
frothy, 2 minutes. Add the cream of tartar and salt. Increase the mixer speed to medium and beat for 2 minutes. Increase the speed to high and continue beating the whites until firm and glossy, but not dry, about 4 minutes more. Gradually sprinkle the remaining 1/2 cup sugar into the whites as they stiffen. Gently fold the whites into the yolk mixture and spoon the batter into the ungreased cake pan. Bake the sponge cake for 50 to 60 minutes, until the top is browned and springy and the cake is baked through. (Test it by inserting a slender skewer; it should come out clean.) Invert the pan and let the cake cool completely before unmolding onto a round platter. Sift the confectioners` sugar over the cake. Cut using a serrated knife.