This simple, light cake is easy to make and provides the layers for our decadent Strawberry Lemon Trifle.
Preheat the oven to 350 degrees. Grease a 9-by-9-inch baking pan with margarine. Dust pan with matzo meal; set aside.
In the bowl of an electric mixer fitted with the whisk attachment, beat egg yolks on medium-high speed with 1/4 cup sugar until thickened, about 4 minutes. Transfer to a large mixing bowl. Wash and dry the mixer bowl and the whisk attachment.
Place egg whites and salt in mixer bowl. Beat on medium speed with whisk attachment until soft peaks form. With machine running, slowly pour in the remaining 1/4 cup sugar; continue beating until stiff and glossy, about 3 minutes. Fold egg whites and orange zest into yolk mixture. Sift potato starch over the mixture, and fold in.
Transfer the batter to the prepared pan. Bake until a cake tester inserted into the middle of the cake comes out clean, 10 to 15 minutes. Transfer pan to a wire rack, and let cool 15 minutes.