For the dough: Dissolve yeast in 1 1⁄4 cups lukewarm water in a large bowl and set aside for about 5 minutes. Combine flour, sugar, and baking powder in the bowl of a standing mixer. Mix in dissolved yeast on low speed, then add lard one piece at a time, increase speed to medium, and continue mixing until dough forms into a ball, about 5 minutes. Remove bowl from mixer, cover with plastic wrap, and allow dough to rise until doubled, about 2 hours. Punch dough down, then knead until smooth and elastic, about 5 minutes. Shape into 16 equal-size balls.
For the filling: Heat oil in a nonstick skillet over medium-high heat. Add scallions and sauté for 1 minute. Add roast pork, sugar, and soy and oyster sauces and continue cooking until scallions have softened and pork is heated through, about 3 minutes. Dissolve cornstarch in 2 tbsp. water in a small cup, then add to pork mixture and cook until sauce thickens, about 1 minute more. Remove from heat and allow to cool slightly.
Place a dough ball in the palm of one hand and, with the thumb of your other hand, make a well in the center. Fill well with about 1 1⁄2 tbsp. pork filling, then seal in by pinching dough closed towards the center. Place a piece of parchment paper over pinched area. Turn bun over and use scissors or a sharp paring knife to make a 1⁄2'' crisscross incision in the center of the bun. Repeat process, filling remaining buns. Keep filled buns covered with a damp towel. Place 8 buns, paper side down, in a bamboo steamer; close tightly with lid. Meanwhile, bring 2 cups water to a boil in a wok over high heat. Fit bamboo steamer into wok and steam until puffed, about 12 minutes. Repeat with remaining buns.