In a large bowl, dissolve yeast in warm milk. Let stand until creamy, about 10 minutes. Add the egg yolk, butter and 2 cups of bread flour; stir well to combine. Stir in the remaining flour, 1/2 cup at a time, beating well after each addition. When the dough has pulled together, turn it out onto a lightly floured surface and knead until smooth and elastic, about 8 minutes.
Divide the dough into 3 equal pieces and roll each piece into a 14 inch long cylinder. Braid the pieces together and place on a lightly greased baking sheet. Cover with a damp cloth and let rise until doubled in size, about 1 hour. Meanwhile, preheat oven to 425 degrees F (220 degrees C).
In a small bowl, beat together egg white and water. Brush risen loaf with egg wash and bake in preheated oven for 20 to 25, until golden.
