In a large mixing bowl, combine 4 cups of all-purpose flour, 2 tablespoons of sugar, and 1 packet of instant yeast. Gradually add 1 cup of warm milk while mixing to form a shaggy dough. Knead for 8-10 minutes until smooth and elastic. Incorporate 2 tablespoons of vegetable oil, cover, and let it rise for about 1 hour.

Heat 2 tablespoons of sesame oil in a skillet over medium heat. Sauté 4 cloves of minced garlic and 1 tablespoon of grated ginger for 30 seconds. Add 1 pound of finely chopped beef chuck, 3 tablespoons of soy sauce, 2 tablespoons of hoisin sauce, and 1 tablespoon of rice vinegar. Simmer for 10-15 minutes until the beef is tender.

Mix 2 tablespoons of cornstarch with 2 tablespoons of water to make a slurry. Stir into the beef filling and cook for another 2-3 minutes until the sauce thickens. Stir in 2 finely sliced green onions and let it cool.

Punch down the risen dough and knead in 1 teaspoon of baking powder. Divide into 14-16 pieces, shape into balls, and flatten into discs. Place 1-2 tablespoons of beef filling on each disc, fold edges over, and pleat to seal.

Line a steamer with parchment paper. Arrange buns with space to expand and let rest for 10 minutes. Steam for 12-15 minutes until puffy. Let sit for 2 minutes before serving.