01 - In a large bowl, combine flour, sugar, instant yeast, baking powder, and salt. Pour in warm milk and vegetable oil. Knead the mixture for 8 to 10 minutes until the dough becomes smooth and elastic. Cover the bowl and let rise for 1 hour, or until doubled in size.
02 - Combine ground pork, Chinese sausage, shiitake mushrooms, onion, garlic, oyster sauce, soy sauce, sugar, white pepper, and spring onion in a mixing bowl. Mix thoroughly until all ingredients are evenly distributed.
03 - Divide the risen dough into 8 equal portions. Roll each portion into a ball, then flatten into a disc measuring approximately 4 inches in diameter.
04 - Place a generous spoonful of filling and a quarter piece of hard-boiled egg in the center of each dough disc. Gather the edges upward and pinch to seal completely, shaping into a round bun.
05 - Place each formed bun on a small square of parchment paper. Let the buns rest for 15 minutes while preparing your steamer.
06 - Arrange buns in the steamer over boiling water. Steam for 15 to 20 minutes until the dough is cooked through and fluffy when touched.
07 - Remove buns from steamer and serve warm.