Combine lukewarm water, yeast, and sugar in a bowl, stir lightly, and let sit for 5 minutes until foamy.

Add flour, baking powder, salt, sugar, and oil to the same bowl and mix until no dry flour remains, then cover and rest for 10 minutes.

Sauté diced onion over medium heat until lightly browned and let cool completely.

Place the dough bowl over warm water (about 113°F / 45°C) and allow it to rise for 30 minutes until slightly expanded.

Punch down the dough, knead for about 3 minutes, divide into 7 portions, shape into balls, and rest for 15 minutes.

Combine cooled onion with pork, salt, soy sauce, sugar, sake, ginger, sesame oil, black pepper, and cornstarch, mixing thoroughly until cohesive.

Flatten each dough ball into about a 4-inch (10 cm) circle and place filling in the center, bring one pair of opposite edges together over the filling and pinch lightly, then bring the opposite pair together to form a diamond shape, pinch the four seams closed, gather the corners at the top and pinch together, pressing the tip down slightly to seal.

Fill a steamer with water and bring it to a boil, place the buns inside, cover, turn off the heat, and let rest for 10 minutes.

Turn the heat back on, wait until the water returns to a full boil, then steam the buns for 15 minutes before removing.