For the filling, mix 200 ml milk with the custard powder and sugar. Bring 1.3 l milk to the boil, pour in the mixed pudding powder while stirring, bring to the boil and pour into a bowl. Cover with foil and let cool down.
For the dough, warm the milk and dissolve the yeast in it. Knead the flour, egg, sugar, butter, salt and yeast milk until the dough starts to bubble. Cover and let rise in a warm place for about 30 minutes.
For the crumble, sieve the flour into a bowl and knead with the sugar, lemon zest, salt, vanilla sugar and butter to make crumble.
Preheat the oven to 150 ° C fan oven. Grease the drip pan of the oven or a 32 x 37 cm baking sheet and sprinkle with breadcrumbs.
Knead the dough, roll it out in the form and press up on the edge with your hands. Mix the pudding with the sour cream and smooth it over the dough, then sprinkle the crumble on top. Bake in the preheated oven for about 35 minutes. Let cool, dust with powdered sugar and cut into slices.