For the dough: Place the flour in a bowl and make a well in the center. Dissolve the crumbled yeast and 1 tablespoon sugar in the milk. Add to the well, cover with a tea towel and let rise for 15 minutes in a warm place.
Add the remaining sugar and milk, the butter, vanilla sugar and egg and mix with the dough hook attachment until smooth. Cover and let rise in a warm place until doubled in bulk, around 35 minutes.
Turn the dough out onto a lightly floured work surface and knead briefly. Divide into 3 portions, shape each portion into a long roll and then braid together, squeezing the ends to seal. Place on a baking sheet lined with parchment paper and let rise again for 5 minutes.
For the egg wash: Whisk the egg yolk and condensed milk until smooth. Brush over the dough and bake in a preheated oven at 180°C / 350°F for 30-35 minutes.