Proof the yeast in the warm water or milk with a pinch of sugar until foamy, about 5-10 minutes.

In a large bowl, whisk together the flour, remaining sugar, baking powder, and salt.

Mix the wet ingredients (proofed yeast, vegetable oil, and vinegar if using) into the dry ingredients until a shaggy dough forms.

Knead the dough for 5-7 minutes until smooth and elastic.

Cover and let the dough rise in a warm place for about 1 hour, or until doubled in size.

If using dried mushrooms, soak them in warm water until softened, then chop finely.

Combine the ground pork with the chopped onion, garlic, sesame oil, oyster sauce, sugar, salt, and black pepper.

Mix in the chopped mushrooms and green onions, adding the cornstarch slurry if you prefer a firmer filling.

Set up your steamer with water and bring to a simmer.

Divide the pork filling into 12 portions and flatten each into a small disc, then place a piece of quail egg or hard boiled egg and a slice of Chinese sausage in the center of each.

Wrap the filling around the egg and sausage to form a compact ball.

Place the filling balls in a heatproof dish.

Add water to the steamer and steam the filling balls for 20-25 minutes, until fully cooked.

Remove from the steamer and let cool slightly before assembling.

Lightly dust a work surface with flour and divide the risen dough into 12 equal pieces.

Flatten each piece into a circle, thicker in the center and thinner at the edges.

Place a steamed filling ball in the center of each dough circle, then gather the edges up and seal tightly.

Place the filled buns seam-side down on squares of parchment paper and let proof for 15-20 minutes.

Steam the buns for 15-20 minutes, adding a splash of vinegar to the steaming water to help keep the buns white.