Pour boiling water over 50 g flour. Stir into a paste and let cool. Add yeast, sugar, and 450 g flour. Knead into a soft dough. Cover and rise until doubled, about 1.5 hours.
Divide ground pork in half. Cook one half in oil until moisture evaporates. Add ginger, garlic, sweet bean sauce, soybean paste, soy sauces, pepper, sugar, and salt. Simmer until liquid evaporates. Mix with remaining raw pork. Refrigerate 30 minutes. Add scallions before wrapping.
Punch down dough. Cut into small pieces. Press flat with your palm — no rolling pin needed. Add filling and pinch edges to seal.
Place buns in steamer. Rest 20 minutes until surface springs back when pressed. Steam 15 minutes. Turn off heat and rest 5 minutes before opening the lid.