Dissolve yeast in lukewarm water. Add 1 cup of flour. Mix thoroughly. Cover with cloth. Let rise 1 hour, until bubbles appear.
Dissolve sugar and vegetable oil in 1/2 cup boiling water. Stir well. Cool until lukewarm. Pour into yeast mixture. Add 3 1/2 cups flour.
Knead dough on lightly floured board until smooth. Put into extra large, greased bowl in a warm place. Cover with damp cloth. Let rise until double in bulk, about 2 hours.
Divide into 2 portions. Remove first portion and knead 2 minutes. Repeat with second. Roll each into roll 12 inches long and 2 inches wide. Cut into 12 pieces (24 total).
Flatten each piece with palm of hand. Roll with rolling pin into 3 inch circles.
Brush with sesame seed oil. Indent middle of circle with chopstick. Fold circle in half so that it becomes a half moon. Crimp edges tightly with fork.
Place each roll on separate square piece of foil on steamer tray. Cover tray with towel. Let buns rise to double in bulk, about 30 minutes. Remove towel.
Steam, tightly covered, over briskly boiling water for 10 minutes. May be prepared in advance. May be frozen. Thaw out in plastic bag and re-steam 10 minutes.