Preheat oven to 180 C / Gas 4. Butter a 24cm round cake tin and sprinkle with breadcrumbs. Shake off excess breadcrumbs.
In a small pan, melt 200g butter and set aside to cool.
Separate eggs. Place the whites in the fridge. Beat yolks with sugar, salt, vanilla sugar, orange flavouring and lemon juice till smooth.
Add quark and custard powder and beat again till smooth. Add cooled butter and beat till smooth.
Beat egg whites until stiff. Gently fold into the quark mixture with a spatula. Tip into the prepared baking tin.
Bake for 40 minutes on the centre shelf of the preheated oven. If the top browns too much, cover it with aluminium foil. Bake for another 10 to 20 minutes, then turn the oven off and leave the oven doors slightly ajar. Wait until the cake is fully cooled, then take out of the oven.