My mother's friend Becka makes this amazing cake. It's so light and fluffy, it's hard to believe there's no flour in it. Prep: 30 minutes; Bake: 50 minutes.
Preheat oven to 350°. Grease a 13- x 9-inch baking dish, and dust with matzo cake meal. Toss apple with cinnamon and 4 tablespoons sugar in a medium bowl. Set aside.
Beat eggs in a large bowl at medium-high speed with an electric mixer. Add remaining 2 cups sugar, and beat 10 minutes or until pale and very thick. Stir in oil and 1/4 cup water. Add 2 cups matzo cake meal and potato starch gradually, and beat to combine.
Pour half of batter into prepared dish. Top evenly with apple mixture, and add remaining batter, spreading gently and smoothing top. Bake at 350° for 50 to 55 minutes or until wooden pick inserted in center comes out clean. Let cool in dish on wire rack. Serve warm or at room temperature.