Stir the yeast into the warm water (or milk), with the salt and sugar.
Set aside for five minutes to dissolve.
Put the flour in a bowl and make a well in it.
Put the eggs and the yeast mixture in the well.
Mix it, drawing in the flour, to make a soft, sticky dough.
Work the dough, lifting and slapping, until it becomes soft and very smooth and elastic. . (This process is a long and difficult job by hand. A mixer with hook attachment is great if you have one.)
Knead the butter to soften it to the same texture as the dough.
Now, work in the butter a piece at a time, again to create a smooth, sticky dough. (Again, the mixer will make that easy.)
Let the dough rise at room temperature in a covered bowl for two hours.
Punch down the risen dough.
Put it back in a bowl, cover, and refrigerate overnight to double.
Shape the bread, let it rise 1-1/2 hours in the pan.
Glaze with yolk and milk wash.
Bake the loaves at 425ºF/220°C for 40 to 45 minutes.