Add all the roux to the pre-ferment dough, then add sugar, salt and butter, mix with a spoon to incorporate.
Add 310g einkorn flour, mix it in at first with two spoons, then turn in onto a very well floured surface, and finish kneading with your hands. Gently, without applying to much pressure, just mostly folding until the dough is somewhat non-sticky. It should feel light and soft and billowy, rather than tight and springy. Stretch it out with your hands.
Roll the dough into a sausage, and immediately transfer it a floured bread pan. Use a large spatula to support it as you transfer. I use a Pullman pan because it's shaped like a loaf of regular sandwich bread (straight up on the sides) and is taller than regular bread pans. The dough will fill 1/4 of the pan. You can shake it gently from side to side a few times to level it.
Cover the bread pan, and leave at room temperature for 5-7 hours. Again, the time will depend on your house temperature, it may be much faster. The dough should rise to about the middle of the pan - so you are looking for it to double (it will continue to rise if you let it, but too much of a rise will cause it to collapse in the oven). Preheat oven to 400ºF (200ºC), you can mist the top of the dough with water. Bake for 20 minutes. Cover with foil if the top is getting too brown, and lower the oven temperature to 350ºF (175ºC) and bake 20 more minutes.
Remove from the bread pan and cool completely before slicing. The crust will be slightly crispy. If you want it to be softer right away, you can brush it with a mixture of 1/2 cup water simmered with 1 teaspoon of sugar and 1 tablespoons of flour for 5 minutes. It will soften the crust. Or, you can wait and store it in a plastic bag overnight and the crust will become soft like the crumb.