In a skillet, heat the oil and saute the scallions, garlic, and ginger over medium heat. Stir in the hoisin sauce and oyster sauce, adding barbecue pork while stirring. Reduce heat to low. Thoroughly combine cornstarch with 2 tablespoons water. Pour over pork mixture and stir until sauce thickens. Set aside. On a floured surface, partially separate breadsticks, leaving 2 thick layers of dough. Roll each into a ball. Working with 1 ball at a time, roll out to make a 3-inch circle. Place about 1 tablespoon of filling in center of each round. Gather dough up and twist to secure tightly. Repeat with remaining dough. Cover with kitchen towel. Arrange cabbage leaves on the bottom of 2 large bamboo steamers. Place 6 pork buns on each steamer. Steam buns for 20 minutes. Serve warm.