Mix 100 ml luke-warm milk, the yeast and 10 g sugar in the bowl of your kitchen machine. Cover and let it stand undisturbed until the yeast is all foamy.
Add the rest of the milk and sugar to the bowl, together with the eggs, flour, butter and salt.
Knead the dough for 5 to 10 minutes until it is soft and comes off the side of the the bowl.
Cover the bowl with plastic wrap and let it stand in a warm spot for 2 hours until the dough doubles in volume.
Half an hour before the dough is ready, preheat your oven to 165 C and start making your filling by warming the chocolate and butter in the microwave or over a water bath.
Once melted, add the the vanilla extract, the cocoa powder and icing sugar and stir until you have a homogeneous mixture (it is quite thick). Let it cool down a bit.
Grease your loaf pan and then roll out your dough into a rectangular shape (more or less 40 cm by 20 cm).
Then spread the chocolate filling evenly over the dough.
Now roll up the dough tightly from one long side to the other and tuck the ends in.
Half the roll lengthwise almost all the way down. Just leave one end together, so it is easier to weave the two attached halves. Make sure the chocolate side is always facing up.
Then transfer your braided bread into your prepared loaf pan.
Mix the egg yolk and cream and brush the surface of your bread with the mixture.
Put your loaf pan into the oven and bake for 35 minutes. You might have to cover the bread during the baking time to avoid that the top gets too dark in colour.
When you remove the bread from the oven, let it sit in the pan for 5 minutes.
Then take the bread out of the pan and let it cool down before you slice it.