For the dough, mix together the low-fat quark, oil, milk, sugar, and vanilla sugar. Mix 250g flour with the baking powder and add it, then stir to combine. Gradually add the remaining flour and knead into a dough.
Bring the poppy seeds to a boil with 3/8l milk and let steep for 10 minutes. Mix the pudding powder with the remaining milk, the egg yolk, and the sugar, add to the poppy seed mixture, and bring to a boil once while stirring. Allow to cool.
Roll out the dough to the size of a baking sheet and spread the poppy seed topping evenly over it.
Knead together the flour, sugar, and (soft) butter with a mixer, let it sit briefly, and then form the streusel. Sprinkle the streusel over the poppy seed topping.
Bake at medium height with convection at 180° for approximately 20 minutes, until the streusel begin to turn brown.