Dissolve the yeast and sugar in 50 ml of warm milk. Leave for 15–20 minutes until bubbles appear.
Add the remaining milk, oil, egg yolk, flour, and salt. Knead until smooth.
Cover and let rise in a warm place until doubled in size.
Roll out the dough, sprinkle with chocolate chips, roll it up, and fold it over. Repeat twice.
Shape into a log, cut into 8–10 pieces, shape into balls, and place in paper liners in a muffin tin. Brush tops with a little milk and let rise 20 minutes.
Bake at 180°C for 20–25 minutes.
Optional: once cooled, fill with Nutella using a piping bag