Mix fresh yeast, sugar, and warm milk. Cover and leave 10–15 minutes until bubbly.
In a bowl, combine sifted flour with the yeast mixture. Add 1 egg yolk and melted butter. Knead until smooth. Cover and let rise 1 hour in a warm place.
Divide dough into 2 equal parts.
Knead cocoa powder into one half until evenly colored.
Roll out plain dough thinly, place cocoa dough on top. Spread Nutella in a 3–4 cm strip on one side, sprinkle chocolate chips, roll up halfway. Cut the other half into strips and twist them. Finish rolling into a log. Place on a lined tray and rest 20 minutes.
Preheat oven to 180°C (356°F). Beat remaining egg yolk with 1 teaspoon milk and brush on top.
Bake 30 minutes.
Cool, then dust with powdered sugar if you like.