In large bowl, beat together egg yolks, half of the sugar, the matzo cake meal, potato flour, lemon rind and juice and vanillin sugar blended. Set aside.
In seperate large bowl and with clean beaters, beat egg whites with salt until soft peaks form. Beat in remaining sugar, 2 tbsp at a time, until stiff peaks form.
With spatula, gently fold one-third of the egg whites into yolk mixture; fold in remaining whites just until no streaks remain. Scrape into ungreased (not non-stick) 10 inch tube pan; smooth top. Bake in centre of 350ºoven until top springs back when lightly touched, about 50 minutes.
Turn pan upside down and let cake hang on legs attatched to pan or on inverted funnel, or bottle, until cooled, about 3 hours. Run along palate knife around center and side of pan, pressing blade against pan and reaching to bottom to loose cake. Unmould onto cake plate.
Citrus Curd: In saucepan, whisk eggs, sugar, lemon rind, lemon juice, and potato flour. Bring to boil over medium-high heat and boil, stirring, for 1 minute. Remove from heat; whisk in margarine. Strain through fine sieve into bowl. Place plastic wrap directly on surface; refrigerate until cold, about 2 hours. Spread 1/3 cup over top of cake; top with strawberries, raspberries and mango. Serve with remaining curd.