Make the sweet levain

1. In a small bowl, mix the sourdough starter, warm milk, flour and sugar until combined. Cover the bowl and let the mixture rise at warm room temperature until doubled in size, about 4 – 8 hours, depending on the temperature in your kitchen.

Make the tangzhong

1. In a small sauce pan, whisk together the flour and milk until smooth. Bring the mixture to a simmer over medium-low heat and continue whisking until the mixture has thickened. To determine when it is ready, run your whisk along the bottom of the pan. It should leave an indentation. This should take 5-10 minutes.
2. Pour the mixture into a small bowl, cover with plastic wrap, and allow to cool at room temperature.

Make the dough

1. In a large mixing bowl, add the flour, sugar, salt, beaten egg, levain, and tangzhong. Using a wooden spoon or Danish dough whisk, mix until it forms a shaggy dough. It will be wet at this point.
2. Mix in the softened butter a little at a time using your fingers. Cover the bowl and let it sit for 30-45 minutes.
3. Perform a stretch and fold to help develop the gluten. Do this by lifting the dough up and folding the dough over itself several times while turning the bowl. The dough will still feel shaggy at this point.
4. Cover, let the dough rest for 2-3 hours. Perform stretch and folds every 30-45 minutes for a total of 3-4 stretch and folds. You should notice the dough becoming more developed with each stretch and fold.
5. Cover the bowl tightly with plastic wrap and place in the refrigerator for 8 hours and up to 24 hours.

Prepare the baking pan:

1. Grease the bottom and sides of a bundt pan, cake pan, or a springform pan and line with parchment paper, if desired.

Shape the dough:

1. Divide the dough into 5 or 6 pieces. Roll each piece into a thin, oblong shape, then, using a sharp object (knife, bench scraper, pizza cutter), start about 2/3s from the designated top and make 1/8” to ¼” cuts.
2. If you’re using a filling, add it now.
3. Begin rolling from the solid top, ending at the bottom, and carefully place it in the prepared pan.
4. Cover the pan, then let the dough rise until doubled in size. This could take several hours, depending on the temperature in your kitchen. Mine took anywhere from 3-5 hours.

Bake the Loaf

1. Towards the end of the rising time, preheat oven to 350 F.
2. If you are using a filling, place the pan on top of a rimmed baking sheet to catch any of the goodness that oozes out.
3. Brush the loaf with milk and bake on the middle rack for 30-35 minutes, rotating the pan partway through for even baking. The loaf should register 190 F. on an instant read thermometer.
4. Let the bread cool in the pan for 10-15 minutes, then remove the loaf to a wire rack to continue cooling. Enjoy the process of eating it, strand by strand.