Warm the milk until lukewarm. Dissolve the fresh yeast in the milk.
Add the flour, sugar, vanilla sugar, salt, lemon zest, butter, and egg to a large mixing bowl.
Add the yeast-milk mixture.
Knead well until a smooth, elastic dough forms.
Cover the dough and let it rise in a warm place for about 80 to 90 minutes.
Knead the dough once more.
Divide the dough into three equal pieces and shape each piece into a long rope.
Braid the three ropes together to form a loaf.
Carefully place the braid on a baking sheet lined with parchment paper.
Cover and let rise again for about 30 to 40 minutes.
Whisk the egg yolk with a little milk and brush the mixture over the braid.
Sprinkle with pearl sugar or sliced almonds.
Preheat the oven to about 200°C / 390°F.
Bake for about 30 minutes. After 20 minutes, cover the braid loosely with aluminum foil if it is browning too quickly.
Let cool before slicing.