Preheat oven to 350°F, with rack in lower third of oven. 

Sift together the potato starch, cake meal, ½ cup of sugar, and vanilla sugar. set aside. 

Place egg whites and cream of tartar in a clean bowl. Beat with the whisk of an electric mixer until the egg whites become frothy, around 1 minute. Add the orange juice, lemon juice, vanilla extract and salt and beat on medium speed until the whites just begin to form peaks, around 3 minutes. Continue mixing on medium speed and gradually add the remaining 1 cup of sugar, 1 tablespoon at a time. raise the speed to medium-high and beat until stiff peaks form and whites are glossy, around 2 minutes. 

Slowly fold the potato starch, cake meal and sugar mixture into the beaten egg-white mixture with a spatula. Then fold in the orange zest. 

Spoon the batter into an ungreased 10-inch tube pan, preferably with a removable bottom. smooth the top, and then transfer to the oven and bake 40-45 minutes, until the cake is springy to the touch and beginning to brown on top. 

Invert the pan immediately onto a cooling rack to keep the cake from collapsing, and cool completely, for around 1 hour. Run a knife around the edges of the pan and the tube to release the cake. 

Once cake has cooled, stir confectioner's sugar and juices, and pour evenly over the top of the cake. Let the glaze set for 30 minutes. 

Recipe published in JOY of KOSHER with Jamie Geller Magazine Pesach 2013 SUBSCRIBE NOW