Place the cake on a baking sheet, brush thinly with strawberry marmalade, and place a baking frame (12x18in (30×45 cm)) around the cake (or place in a deep cake pan).
Preheat oven to 320 °F (160 °C).
Wash the strawberries and set aside about 15 pretty ones with their stems.
Halve the rest (without stems).
Mix the quark with sour cream, confectioner’s sugar, vanilla sugar, cornstarch, lemon and orange zest, and a dash of salt until creamy.
Mix in the egg yolks.
Begin beating the egg whites, add the granulated sugar, and beat to stiff peaks.
Fold in carefully.
Put the strawberries on the cake and spread the quark mixture on top.
Bake for about 35 minutes.
Remove from the oven, let cool, and serve.
Before serving, garnish each slice with a half strawberry (with the green part).
