1. Add sponge ingredients to the bowl of a stand mixer.
2. Mix with a spatula, cover with plastic wrap and let it stand for 45 minutes.
3. Then add the dough ingredients except for the butter, mix with the mixer with the dough hook on medium speed until well combined.
4. Then increase the speed to medium-high and continue mixing until the dough pulls away from the sides and looks shiny and elastic.
5. This should take 10 to 13 minutes.
6. Scrape down the sides every 4 to 5 minutes.
7. Add the butter very slowly and gradually!
8. 1 tbsp at a time while the mixer is running, letting it mix in completely before adding the next spoonful.
9. It should take at least 13 to 15 minutes for this.
10. Then mix for 5 to 7 more minutes afterward until the dough passes the windowpane test.
11. I had to add a bit more flour as I think my kitchen was too hot so the dough wouldn’t come together.
12. But that worked like a charm, and it came together beautifully.
13. Cover the bowl with plastic wrap and let it rise for about an hour or until it doubles in size.
14. Then turn it out on a lightly floured surface and punch down.
15. Divide the dough in half, then cut each half into 6 equal pieces.
16. Flatten each piece in the shape of a rectangle, then roll them up from the short side.
17. Grease two loaf pans (8″x5″) with butter.
18. Place 6 pieces in each pan, seam side down.
19. Cover with plastic wrap and allow to rise until doubled in size, about an hour to 90 minutes. I only had 45 minutes, and it worked perfectly.
20. Brush with egg wash and sprinkle lightly with salt.
21. Preheat the oven to 375°F/190°C.
22. Bake until deeply golden brown on top and baked through, about 30 minutes.
23. I tented mine with foil after about 15 minutes to prevent it from burning.
24. Let it cool for 5 minutes, then turn out onto a cooling rack to cool completely.

A few notes:

1. Be prepared to hold down your stand mixer on the counter!
2. This dough is sticky and tough work to mix, and it pushes the mixer to its limit.
3. Oh, and I most definitely do not suggest trying to make this without a stand mixer – it is NOT easy to mix and knead!!
4. I recommend measuring the weights rather than using the cup sizes.
5. This dough benefits from precision.
6. You can make this the day before you need it, put it in the fridge after
   the first rise – just punch it down right in the bowl, recover with plastic,
   and stick it in the fridge.
7. The extra time is supposed to really help the flavor develop, but I found it to be delicious even on the same day.
8. Proof until doubled before baking, up to 2 and a half hours.
9. This was by far the most delicious bread I’ve ever made!
10. It’s a lot of work, but so, so good. I couldn’t believe how fluffy and light the loaf was – I mean, look at how high it rose in the pictures! So delicious too.
11. Worth the effort, I’d say!

Original Source:  Delish