To make dough: Dissolve sugar in warm water (water should be as hot as your finger can stand to touch). Add yeast. Let stand 10 minutes as yeast foams and rises.
Sift flours together twice. Place in a large bowl. Add shortening, then slowly add the yeast mixture, incorporating it into the flour gradually. Form into a ball.
Turn onto a floured surface and knead 5 to 7 minutes, until smooth and elastic. Add more flour or water if necessary. Place in a clean bowl and cover with a damp cloth. Let rise in a warm place for 2 to 4 hours, until at least doubled in bulk.
To make filling: Heat vegetable oil in a wok. Add remaining ingredients, except the cornstarch slurry, and stir-fry until hot. Bring to a boil, then stir in slurry to thicken. Cool.
To assemble bao: Remove warm dough from bowl and knead on a floured surface 5 to 7 minutes, adding more water if too dry, or more flour if too wet. Form into a long roll and divide into 24 equal portions. Lightly oil a Chinese knife or cleaver. Place a section of dough cut-side down on a flat surface and pound with the flat side of the knife. Then press down on the dough with the knife and turn the knife clockwise to form the dough into a thin circle, about three inches in diameter. Use the knife to lift the dough and place it in your hand.
Place a heaping tablespoon of filling into the circle, gather up the edges and pinch closed in the center. Place on a square of paper. (Dough is easier to work with when warm. Beginners may wish to work with half the dough at a time, keeping the remainder in a warm place.).
Let filled buns rise in a warm place 15 minutes.
Oil steamer baskets and arrange buns 1/2-inch apart. Fill a wok 75 percent with water and bring water to a boil. Steam buns 15 minutes over high heat. If buns are in two stacked trays, switch the trays midway. Serve hot.