Sift the flour into a bowl and form a well in the center. Add yeast, a little sugar, and some of the lukewarm milk, then let it activate briefly.
Add the remaining ingredients and knead into a smooth dough (8–10 minutes). Cover and let rise for 60–90 minutes until doubled.
For the custard, heat the milk. Mix egg yolk, sugar, and cornstarch, add hot milk, cook until thick, then stir in butter and let cool.
Roll the dough into a rectangle (approx. 30 x 40 cm) and spread the custard evenly on top.
Sprinkle chocolate chips over the custard, fold the dough, then lightly roll out again.
Cut into strips, twist each strip and connect the ends to form a spiral.
Place on a baking tray, let rise briefly, add a few more chocolate chips and brush with egg yolk-milk mixture.
Bake at 180°C (350°F), top and bottom heat, for 18–22 minutes until golden.
