If using frozen lingonberries, let them thaw slowly in refrigerator. Bring the quark and the butter to room temperature.
Prepare the dough for the bottom crust. Stir the quark until smooth, add the soft butter. Mix thoroughly. Finally add the flour mixing it in gently by hand. Stir just until combined, do not overmix.
Cut a disk of parchment paper and place on the bottom of a small springform cake pan (Ø 20 cm). Using your hands, lightly press the soft dough onto the bottom and the sides (about 3 cm up) of the pan.
Although the amount of dough may seem inadequate, just keep spreading it patiently. The crust will be fairly thin. Place the pan in refrigerator.
For the filling, mix the quark until smooth. Mix in the sugar and the vanilla sugar, then the flour, the cream and the egg. Mix until smooth.
Gently mix the lingonberries with the brown sugar. Spread the mixture on the bottom crust. Pour the quark mixture evenly on top. Bake for about 30-40 minutes
Bake the cake in the lower part of the oven, preferably using the bottom heat. Otherwise the bottom crust may easily remain half-baked and soggy. Using a glass pie dish will also help to determine when the bottom crust is cooked enough. The centre of the filling will remain slightly wobbly.
In room temperature, let the cake cool thoroughly in the pan.
When the centre of the cake is firm enough, remove the cake from the pan, cut in slices and serve. The cake may also be served chilled.