In a small bowl, dissolve yeast in warm water. In a large bowl, combine sugar, salt, yeast mixture and 3 cups flour; beat on medium speed 3 minutes. Stir in enough remaining flour to form a soft dough (dough will be sticky)., Turn onto a floured surface; knead until smooth and elastic, 6-8 minutes. Place in a greased bowl, turning once to grease the top. Cover with a kitchen towel and let rise in a warm place until doubled, about 30 minutes., Preheat oven to 500°. Punch dough down. Divide and shape into 6 balls. Place 3 in. apart on 2 baking sheets that have been generously sprinkled with cornmeal or greased. Cover with a kitchen towel; let rise in a warm place until doubled, about 15 minutes. Spray loaves with water; if desired, generously sprinkle with sesame seeds. Using a sharp knife, score surface with shallow cuts in an "X" pattern. Bake 2 minutes. Reduce oven setting to 425°. Bake until golden brown and internal temperature reaches 190°-200°. Remove from pans to wire racks to cool., Cut a thin slice off the top of bread. Hollow out bottom portion of loaf, leaving a 1/2-in. shell. Discard removed bread or save for another use, such as croutons.