In a large mixing bowl, dissolve the yeast in warm water. Add the egg yolks, sugar, and vanilla sugar. Whisk until well mixed.

Add the flour, melted butter, and salt. Stir with a wooden spoon for 2 minutes, then let the dough rest for 5 minutes. Stir for another 4 minutes or use a dough hook on low speed.

Turn the dough onto a floured surface and knead for 1-2 minutes, adding more flour if sticky, until soft and manageable. Form it into a ball.

Place the dough in a large oiled bowl, cover with plastic wrap, and refrigerate overnight or for up to 4 days.

When ready, take the dough out, divide into 2 equal parts for 2 loaves, then each part into 5 pieces.

Roll each piece into a rope about 10 inches long. Pinch one end of the ropes together to start braiding.

Braid by crossing rope 1 over 3, 2 over 3, and 5 over 2. Repeat until finished and pinch the ends together.

Transfer to a lined baking sheet, brush with egg wash, and let rise for 1 hour in a warm place.

After the first rise, brush again with egg wash, sprinkle with seeds if desired, and let rise for another 30 minutes to 1 hour.

Preheat oven to 350°F (175°C). Bake for 30-40 minutes until golden and hollow when tapped. Cool completely before slicing.