Dissolve yeast and sugar in 1/3 cup warm water in a small bowl; let stand 5 minutes.
Place flour and 1/2 teaspoon salt in a food processor; pulse 2 times or until blended. With processor on, slowly add yeast mixture and 2 teaspoons oil through food chute; process until dough leaves sides of bowl and forms a ball.
Turn the dough out onto a lightly floured surface. Sprinkle baking powder over surface of dough, and knead lightly (about 2 minutes).
Place dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place (85°), free from drafts, 1 hour or until doubled in bulk.
Combine sherry, 1/2 cup water, and chicken in a medium saucepan; bring to a boil. Cover, reduce heat, and simmer 25 minutes or until chicken is done. Remove chicken from cooking liquid, and let cool. Set aside 1/2 cup cooking liquid, and let cool completely. Discard remaining cooking liquid. Remove chicken from bones, and shred chicken.
Combine 1/2 cup cooking liquid and cornstarch; stir well, and set aside. Heat 2 teaspoons oil in a medium nonstick skillet over medium heat. Add onion; sauté 3 minutes or until tender. Stir in curry powder, gingerroot, and garlic; cook 30 seconds. Add cornstarch mixture; bring to a boil, and cook for 1 minute, stirring constantly. Remove from heat; stir in shredded chicken and 1/2 teaspoon salt. Set aside.
Punch dough down, and turn out onto a lightly floured surface. Divide dough into 14 equal portions. Working with 1 portion at a time (cover remaining portions to keep dough from drying out), roll each portion into a 4-inch circle. Spoon about 1 1/2 tablespoons of chicken mixture onto center of each circle. Bring edges of dough to center, pinching edges together to seal and form a bundle. Cover and let rise 45 minutes or until puffy (dough will not double in size).
Line a bamboo steamer with steamed cabbage leaves. Place buns, seam sides down, 1/2 inch apart in steamer; cover with steamer lid. Add water to a large skillet to a depth of 1 inch; bring to a boil. Place steamer in skillet, and steam buns 15 minutes. Remove buns from steamer.
Note: To bake buns, preheat oven to 350°. Place buns, seam sides down, on a baking sheet coated with cooking spray. Combine 1 egg and 1 tablespoon water; brush over buns. Bake at 350° for 18 minutes. Baking will give the buns a crustier texture; steaming makes the dough moister and a little sticky.