In a small bowl, dissolve yeast in warm water. Let stand until creamy, about 10 minutes.
In a large bowl, stir together the flour sugar and salt. Make a well in center of the bowl and mix in the eggs and yeast mixture. Beat well until the dough has pulled together, then turn it out onto a lightly floured surface and knead until smooth and supple, about 8 minutes.
Flatten the dough and spread it with one third of the butter. Knead this well. Repeat this twice to incorporate the remaining butter. Allow the dough to rest for a few minutes between additions of butter. This process may take 20 minutes or so. Lightly oil a large bowl, place the dough in the bowl and turn to coat with oil. Cover with plastic wrap and let rise in a warm place until doubled in volume, about 1 hour.
Deflate the dough, cover with plastic wrap, and refrigerate 6 hours or overnight. It needs time to chill in order to become more workable.
Preheat oven to 400 degrees F (200 degrees C). Lightly grease two 9x5-inch loaf pans (see Cook's Note to make rolls). Beat the egg yolk with 1 teaspoon of water to make a glaze.
Turn the dough out onto a lightly floured surface. Divide the dough into two equal pieces, form into loaves and place into prepared pans. Cover with greased plastic wrap and let rise until doubled in volume, about 60 minutes.
Brush the loaves or rolls with the egg wash. Bake in preheated oven until a deep golden brown. Start checking the loaves for doneness after 25 minutes, and rolls at 10 minutes. Let the loaves cool in the pans for 10 minutes before moving them to wire racks to cool completely.
