In a large bowl, combine the active starter, warm milk, and sugar. Mix until the starter dissolves.
Stir in melted butter, egg, vanilla, and salt.
Gradually add the flour, mixing until a sticky dough forms. You can use a stand mixer with a dough hook or mix by hand.
Knead the dough for 8–10 minutes (or until smooth and elastic). If it’s very sticky, add small amounts of flour, a tablespoon at a time.