Add milk and yeast to a cup and stir well with a fork. In a large mixing bowl, combine flour, sugar, and baking powder. Add the milk mixture and stir with a spatula until well combined, then add the corn oil. Knead the dough by hand or in a stand mixer with a dough hook attachment.
Knead the dough at a slow speed for 8 to 10 minutes until smooth. Then, cover the dough with plastic wrap and let it rest for 5 minutes. Meanwhile, prepare the scallion filling by combining chopped scallions, salt, and baking soda in a bowl. Stir well and set aside.
Transfer the dough to a floured surface and flatten it into an 8 x 16-inch (20 x 40 cm) rectangle using a rolling pin. Brush oil over the surface and evenly distribute the seasoned scallions on top with a spoon. Fold the dough from left to right into three segments (or roll it up tightly), then divide it into 10 equal pieces.
Take one piece of dough and use a chopstick to press down in the middle. Remove the chopstick, then gather the two ends together and press down with your thumb and index fingers. Repeat this process to shape the remaining buns.
Place each bun on a small piece of parchment paper and transfer them to a steamer, ensuring there’s enough space between each bun so they don’t stick together. Cover the steamer lid and let the buns rise for 50 to 60 minutes, or until they have expanded in size.
Fill the steamer's bottom layer with hot water. Turn on high heat and steam the buns for 10 minutes. Afterward, turn off the heat and let the buns sit inside the steamer for 3 minutes before removing the lid. Serve immediately.