Combine yeast, water and sugar in a large mixer bowl. Set aside for 5 to 10 minutes to allow yeast to foam. Add 1 cup flour, butter and salt and mix at medium speed for 2 minutes using paddle attachment on electric mixer until well blended. Add two eggs and enough remaining flour to make a soft dough. Switch to dough hook and knead on high speed 4 to 6 minutes until dough is smooth and elastic.
Place in large bowl sprayed with nonstick cooking spray. Spray dough with nonstick cooking spray and cover. Let rise in a warm place until doubled in size; about 30 to 45 minutes.
Punch dough down. Remove dough to lightly floured surface; pat dough and shape into an approximate 10 x 6-inch rectangle. Divide into 3 equal strips by making two cuts with a sharp knife or pizza cutter vertically starting 1-1/2-inches from the top; keeping one end of dough intact. Braid strips and place into a 9 x 5-inch loaf pan sprayed with nonstick cooking spray. Cover; let rise in warm place until doubled in size, about 30 to 45 minutes.
Beat egg yolk with 1 teaspoon water; brush over loaves. Sprinkle with poppy seed, if desired. Bake at 375oF for 25 to 30 minutes or until done. Let cool on wire rack 5 minutes before removing from loaf pan.
Bread Machine Option – Use this order to add ingredients to bread machine bin. First, place sugar and salt. Second, add butter, eggs water. Third, add flour and yeast. Select dough cycle. When complete, follow the shaping and baking instructions from above.