In the bowl of a stand mixer fitted with a dough hook, combine flour, sourdough starter, sugar, salt, eggs, and milk. Knead for approximately 10 minutes until ingredients are well combined. Allow the dough to rest, then gradually incorporate the softened butter, kneading between additions. Continue kneading for about 5 minutes until the dough achieves a smooth yet slightly tacky consistency. Cover and let rest in a warm, draft-free environment for 2 hours before transferring to the refrigerator overnight.

The next day, turn the chilled dough onto a lightly floured surface and roll it into a 10x18 inch (25x45 cm) rectangle, approximately ¼ inch thick. Evenly spread the chocolate or hazelnut cream over the surface, leaving a ½ inch (1 cm) border. Sprinkle with chopped nuts and chocolate chips.

Prepare two 8–9 inch (20–22 cm) loaf pans, or alternatively a 9½ inch (24 cm) bundt or tube pan, by greasing and flouring them. Roll the dough tightly along the long edge into a log, then slice it lengthwise and twist the two halves to form a braid. Divide if needed and transfer to the prepared pans, tucking the ends neatly.

Cover the pans and allow the dough to rise in a warm, draft-free area until doubled in size, which will take between 3 to 6 hours.

Preheat the oven to 375°F (190°C). Whisk the egg and water together to create the egg wash, then brush it over the risen loaves. Bake for 25 minutes, then reduce the temperature to 350°F (180°C) and bake for an additional 5–10 minutes until the top is deeply golden. Transfer immediately to a wire rack to cool fully before slicing.