Mix the flour, sugar, vanilla sugar, lemon zest, orange baking flavor and salt in a larger bowl. Separate one egg, keep the egg whites for the icing in the refrigerator until the next day. Then add the 2 whole eggs, the egg yolk, the lukewarm milk, initially only 100 g butter and the pre-dough and knead everything until a firm dough is formed that turns away from the bowl wall. A food processor would be ideal for this, as it takes a while until everything is well kneaded and also requires a little force ..... but I don't have one, and so I had to do it - with a little effort - with a hand mixer ... That was it then "fitness" ... ;-)), but it is quite feasible. Then put the finished dough in a very large (at least 5 l), lightly oiled bowl, seal it tightly and absolutely airtight and place in a warm place. It has to stay there overnight (see foreword) and until the next morning. After resting the dough, it should have at least tripled its volume. Do not put the remaining 100 g butter in the refrigerator overnight so that it is not too hard the next day.