Heat milk till lukewarm. Then mix 3tablespoons milk with the yeast, 2tablespoons flour and 1tablespoon sugar. Let this starter rest for 10 minutes.
Mix the rest of the milk with the 3 egg yolks, the rest of the sugar and the salt.
Add the yeast mixture and the rest of the flour. Make a very hard dough out of it, adding more flour if necessary.
Work 6tablespoons of the butter into the dough, making a very smooth, even texture. Let the dough rise for about 1 hour. Preheat oven to 375°F.
Divide the dough into 4 equal long strands, cutting each the same length as the baking sheet that you plan to use.
Take 2 strands of the dough and braid them neatly. Make a second braid with the other 2 strands. Butter the braids with 2tablespoons of the butter, using a pastry brush, and let them rest in a moderately warm place for 30 to 35 minutes.
Whip the egg white till stiff and brush tops of braids with it. Sprinkle tops with ground walnuts.
Use remaining butter to grease the bottom of a baking sheet, and place the prepared braids on it.