In a large bowl, mix the sourdough starter, eggnog, flour, egg, sugar, salt, nutmeg, and vanilla until a sticky dough forms. Rest for 30 minutes.

Add softened butter and knead the dough until smooth and elastic (8–10 minutes).

Cover and bulk ferment at room temperature for 4–6 hours, doing 3 stretch-and-folds every 30 minutes for the first 90 minutes.

Shape into a ball or place in a loaf pan once doubled in size.

Cover and cold-proof in the refrigerator for 8–12 hours.

Preheat oven to 375°F (190°C). Bake covered for 20 minutes, then uncovered for 20–25 minutes until golden and sounds hollow.