Mix active starter, milk, water, egg, sugar, butter, and salt in a large bowl. I recommend feeding your starter in the morning so it will be active and ready in the evening.
Add flour gradually until you have a soft, slightly tacky dough.
Knead 5–7 min by hand or 3–5 min in a stand mixer until smooth.
Cover and let rise at room temp for 2–3 hours until slightly puffy.
Refrigerate overnight (8–12 hours).