In a large mixing bowl cream the butter with the remaining sugar. Add the egg yolks one at a time, beating well after each addition. Add the yeast mixture, remaining milk, 4 cups of flour and the salt; mix well. Add the remaining flour, 1/2 cup at a time, stirring 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. Lightly oil a large bowl, place the dough in the bowl and turn to coat with oil. Cover with a damp cloth and let rise in a warm place until doubled in volume, about 1 hour.