For the dough: Combine yeast in 1 1⁄4 cups water heated to 115° in a bowl; let sit until foamy, about 10 minutes. Combine flour, sugar, and baking powder in the bowl of a stand mixer fitted with a paddle. Add yeast mixture, and mix on low speed; 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 let sit until doubled in size, about 2 hours. Knead dough until smooth and elastic, about 5 minutes. Shape into 16 equal-size balls.
For the filling: Heat oil in a 10" nonstick skillet over medium-high heat. Add scallions; cook for 1 minute. Add roast pork, soy and oyster sauces, and sugar; cook until scallions have softened and pork is heated through, about 3 minutes. Dissolve cornstarch in 2 tbsp. water in a small cup, add to pork mixture, and cook until sauce thickens, about 1 minute more. Remove from heat, and let cool.
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; seal by pinching dough closed toward the center. Place a 2"-square piece of parchment paper over pinched area. Turn bun over, and use scissors to make a 1⁄2" crisscross incision in the center of the bun. Repeat process, filling remaining buns and placing on parchment paper squares. Keep filled buns covered with a damp towel. Place 8 buns, paper side down, in an 11" bamboo steamer; close tightly with lid. Meanwhile, bring 2 cups water to a boil in a 14" flat-bottomed wok over high heat. Fit bamboo steamer into wok, and steam until puffed, about 12 minutes. Repeat with remaining buns.