1. Feed your sourdough starter 8–12 hours before starting and ensure it doubles and passes the float test

2. Whisk warm milk, sugar, vanilla, eggs, and sourdough starter in a stand mixer bowl

3. Add both flours and mix on low until a cohesive but slightly sticky dough forms

4. Gradually add softened butter pieces and knead on medium speed until the dough is glossy, elastic, and smooth

5. Transfer the dough to a lightly oiled bowl, cover, and ferment in a warm place for 4–6 hours until doubled

6. Gently flatten the dough, sprinkle chilled chocolate chips, and fold corners toward the center to distribute evenly

7. Shape the dough into a smooth ball and place it in a tall panettone mold or lined pan

8. Cover and proof for 6–10 hours until the dough nearly reaches the top of the mold and feels light

9. Preheat the oven to 350°F (175°C) and score a shallow cross on top or add a small pat of butter

10. Bake for 35–40 minutes until the internal temperature reaches 190°F (88°C), covering with foil if browning too quickly

11. Immediately invert the loaf using skewers to suspend it upside down until completely cool

12. Slice to reveal the airy, tender crumb dotted with melted chocolate chips and serve slightly warm