In a small bowl, combine 1/2 cup warm water, yeast and 1/2 teaspoon sugar; let stand for 5 minutes. In a large bowl, cream butter and remaining sugar. Beat in eggs. Add 4 cups flour, salt and remaining warm water; beat until smooth. Beat in yeast mixture. Stir in enough remaining flour to form a soft dough.
Turn onto a floured surface; knead until smooth and elastic, about 6-8 minutes. Place in a greased bowl, turning once to grease top. Cover and let rise in a warm place until doubled, about 1 hour.
Punch dough down. Turn onto a lightly floured surface; divide into six portions. Divide each portion into five pieces. Divide each piece into three balls. Place three balls in each greased muffin cup. Cover and let rise until doubled, about 1 hour.
Beat egg yolk and cold water; brush over dough. Sprinkle with sesame seeds. Bake at 350° for 15-20 minutes or until golden brown. Remove to wire racks.