Prepare the dough: In a large bowl, combine the sourdough starter, warm milk, sugar, salt, butter, and eggs. Mix well. Gradually add flour, one cup at a time, until the dough comes together. Knead the dough for 8-10 minutes, or until smooth and elastic. Add flour as needed.
- Let the dough rise: Cover the dough and let it rise for 4-6 hours at room temperature, or refrigerate overnight for a slow rise.
- Prepare the filling: In a small bowl, combine softened butter, brown sugar, cinnamon, nutmeg (if using), and a pinch of salt. Stir until smooth.
- Shape the dough: After rising, roll the dough into a 12×16-inch rectangle. Spread the cinnamon filling over the dough, leaving a small border.
- Roll and twist: Roll the dough tightly from the longer side and slice it in half lengthwise. Twist the two halves together, exposing the cinnamon swirl layers. Place the twisted dough in a greased 9×5-inch loaf pan.
- Second rise: Let the dough rise for 1-2 hours until it rises above the pan.
- Bake: Preheat the oven to 350°F (175°C). Bake the twist bread for 30-35 minutes until golden brown and sounds hollow when tapped.
- Glaze and serve: Mix powdered sugar, milk, and vanilla extract to make a glaze. Drizzle over the cooled bread. Slice and serve warm.