In a large mixing bowl, dissolve the yeast in warm water, let sit for 5 minutes.
Add butter, surgar, salt, eggs, and 3 cups of flour to the yeast mixture. Beat on medium speed for 3 minutes.
Stir enough remaining flour to form a soft dough. Turn out onto a lightly floured surface; knead until smooth & elastic, anout 6-8 minutes.
Place in a greased bowl, turning once to grease all sides. Cover and let rest in a warm place until doubled, about an hour.
Punch down dough. Turn onto a lightly floured surface; divide dough into four pieces; shape each piece into an 18" rope.
Place ropes parallel to each on a greased baking sheet. Beginning from the right side, braid dough by placing the first rope over the second, under the third and over the fourth. Repeat 3 or 4 times, beginning each time from the right side. Pinch ends to seal and tuck under. Cover with tea towel and let rest until doubled in size, about 45 minutes.
Beat egg yolk & water and brush over braid. Bake at 350 degrees for 20-25 minutes or untilgolden brown.