Combine yeast with sugar and warm water and leave fifteen minutes until it bubbles up.

In a small pan, heat up milk and butter until they combine, and then turn off and leave to cool down to lukewarm.

In a larger bowl, combine flour, chocolate, salt, and sugar. Add milk and butter and knead into a ball of dough as if you are making bread. Leave aside for hour to an hour and a half to rise.

Flour a working surface and transfer the dough onto it. Divide in two and work (knead) each ball a little more. With a rolling pin, roll one ball into a rectangle, the thickness of a small finger. Take 4 ounces of Nutella and spread over the dough equally. Roll the dough in until you have a loaf.

Repeat with the other dough ball and transfer into a baking pan.

Mix the egg yolk and paint the top of each strudel. Leave aside for 30-45 minutes until it rises again.

Heat oven to 390°F. Transfer the pan to the oven and bake for 10 minutes. Turn the pan, and cover with foil. Lower to 350°F, and bake 25 minutes. Turn the oven off and leave in another 5 minutes.

Cool for 15-20 minutes before eating.