For the Levain/Starter:

1. Mix together all of the levain ingredients in a jar. Let sit at room temperature to mature and rise, about 8 hours.

For the Final Dough:

1. Place all of the dough ingredients (including the levain) except for the salt and butter in the bowl of stand mixer fitted with the paddle attachment. Mix on low speed until it comes together in a shaggy dough. Let rest (autolyse) for 30 minutes.
2. Switch to the dough hook. Sprinkle in the salt and knead for 3-5 minutes, or until the dough starts to come together.
3. Add the butter gradually, mixing well after each addition. It will start out greasy, but the butter will eventually absorb into the dough. Continue kneading until the dough passes the windowpane test, about 10 minutes.
4. Place the dough in a lightly greased bowl, then turn to grease all sides of the dough. Cover tightly and allow to rise at room temperature for 2 hours. Place in the refrigerator to rise overnight.
5. The next day, divide the dough into 3 pieces and form into balls. Place on a lightly floured surface, cover with a damp tea towel, and allow to rest for 1 hour.
6. Roll each ball into an elongated oval, then roll each one up jelly-roll-fashion. Allow to rest for 10 minutes. Roll them out into an oval again along the seam, then roll each one up like a jelly roll once more. Transfer the rolls to a buttered 9×5-inch loaf pan. Cover with a damp tea towel and allow to rise at room temperature for about 6 hours. At that point the dough should have puffed up and filled the pan about 2/3-3/4 of the way.
7. minutes before the loaf is ready to bake, preheat your oven to 400 degrees F. Place the risen loaf into the hot oven and bake for 20 minutes. Lower the oven temperature to 350 degrees F and continue to bake for 15-20 minutes, or until the loaf reaches an internal temperature of 195-200 degrees F.
8. Transfer to a wire cooling rack and allow to cool completely before slicing (if you can).