1. Mix flour, milk, and eggs in a bowl until combined, then rest for 30 minutes for autolyse.

2. Stir in sourdough starter, sugar, salt, and vanilla, then knead until the dough begins to form.

3. Add softened butter in small portions, kneading until smooth and elastic.

4. Place dough in an oiled bowl, cover, and bulk ferment for 4–6 hours at room temperature until doubled.

5. Refrigerate the dough for at least 8 hours or overnight for a cold rest.

6. Prepare filling by mixing melted butter, cocoa powder, brown sugar, chopped chocolate, salt, and chopped hazelnuts.

7. Roll chilled dough into a 12 x 16 inch rectangle, spread filling evenly, then roll into a log.

8. Slice the log lengthwise, twist the two halves together with cut sides up, and place in a lined loaf pan.

9. Proof the shaped babka for 3–4 hours at room temperature until risen above the pan edge.

10. Preheat oven to 180°C (350°F), brush with egg wash, sprinkle nuts or sugar, and bake for 35–40 minutes.

11. Cool loaf for at least 20 minutes before slicing to allow chocolate layers to set.