Prepare the dough: Mix Hong Kong flour, all purpose flour, sugar, instant dry yeast, baking powder, and milk to form a dough. Knead until smooth and elastic.
Make black sesame filling: Roast black sesame seeds until fragrant. Grind them finely and mix with sugar, brown sugar, salt, and softened butter to form a rich filling.
Prepare mochi filling: Combine glutinous rice flour, sugar, and milk or water. Microwave the mixture until it forms a sticky mochi dough. Portion into 6-8g pieces.
Assemble baos: Dust hands with cornstarch to prevent sticking. Flatten dough balls, place a portion of black sesame filling and a piece of mochi in the center. Seal the dough tightly by pinching the seam to prevent leakage.
Cook baos: Instead of steaming, cook the baos using a microwave method to save time. Ensure the baos are fluffy and cooked through.