In a large mixing bowl, add flour, sugar, and baking powder. Add milk mixture and stir to combine well with a spatula before adding corn oil. Knead the dough by hand or in a stand mixer with a dough hook attachment.
Transfer the dough to a floured surface, then flatten and shape the dough into an 8 x 16-inch rectangle with a rolling pin. Brush oil and evenly distribute the seasoned scallion on top with a spoon. Fold the dough from left to right in three segments (or roll it up tightly), then divide it into 10 equal pieces.
Take one piece, use a chopstick, and press down in the middle. Remove the chopstick. Gather two ends together and press down with your thumb and index fingers. Repeat the same to shape the remaining buns.
Place each bun on a small piece of parchment paper and transfer them to a steamer. Make sure to leave enough space between each bun so they don't stick together. Cover the lid of the steamer and let rise for 50 to 60 minutes, or until the buns expand in size.