Banh Bao, Vietnamese Steamed Pork Buns

Ingredients

• 2½ tsp active dry yeast
• 1 cup warm water or milk
• ¼ cup sugar
• 4 cups all-purpose flour
• 1 tsp baking powder
• 1 tsp salt
• 3 tbsp vegetable oil
• 1 tbsp vinegar optional
• 1 lb ground pork
• 1/2 cup finely chopped onion
• 2 cloves garlic minced
• 1 tsp sesame oil
• 1 tbsp oyster sauce
• 1 tsp sugar
• 1/2 tsp salt
• 1/4 tsp ground black pepper
• 1/2 cup finely chopped mushrooms
• 1/4 cup chopped green onions
• 1 tbsp cornstarch optional
• 1 dozen quail eggs or 3 large hard boiled eggs cut into 4ths
• 3 Chinese sausage links sliced