Make the levain, let it become very active, 4-6 hours.
Build the dough, in the stand mixer, with dough hook (not paddle as in above website). Only add the butter at the end, in phases.
250g all purpouse flour
250g bread flour
187g eggs (3 eggs + 1 yolk + whatever is required from last egg white)
200g levain (quantity above with evaporation)
Mix well, ~10min in stand mixer (while adding in the butter).
Let rise for a good 6 hours in warm area, won’t quite double in size. Then, overnight in the fridge.
Next morning, use cold dough and some flour to make ~18 portions of 65g in tight little balls. They are quite sticky, it takes some time to get the hang of making tight dough balls that small!
Place far enough apart on generously floured trays so that they can spread out a double in size. They are very delicate at the end, and it will be very difficult to unstick them from the trays!
Brush with water and let rise for ~12 hours. Cover if not is a very humid environment (e.g. proofing oven). Can put in the fridge after the proofing, remove from fridge and do the final 2 hours of proofing in the morning.
Place (back) in the fridge for 30min as you prepare the oil, ~350F. Cooling them down makes them slightly less super difficult to pick up. Place in the oil with the side that was proofing on top downwards, into the oil (i.e. flip them). 90 seconds on each side.
Place on paper towel to remove the oil for ~1min, then coat with sugar. Can fill them with creme patissiere (see below, from same website).
4 egg yolks
4g vanilla extract
Bring milk to just under boil (don’t boil!) in a saucepan.
Mix all the other ingredients into a bowl.
Slowly pour milk into the other ingredients, whisking. Then pour back into the saucepan, and gently bring to a boil on medium. As soon as it starts to boil, it will be thick and make big bubbles, remove it from heat and pour it through a strainer into a clean bowl.
Cover the surface with plastic (push onto the surface to prevent skin from forming), and refrigerate until needed.
Make the dough, let it rise. Roll it out to a rectangle ~1cm thick (such that the width is less than the length of the loaf pan; allow for a good tolerance). Brush melted butter, sprinkle with filling, roll it up, gently and carefully pinch the seams closed. Place in greased loaf pan, seam side down, egg wash, and rise again until double.
Egg-wash + oven at 350F for 35-45min.
Remove from loaf pan when cooling, after 10min or so.