Whisk the flour, cornstarch, and salt together in a large bowl to combine.

Warm the soy milk to approximately 110°F—it should feel warm to the touch but not hot. Stir in the sugar and yeast and let the mixture sit for 10 to 15 minutes until a thick foam forms on the surface, then stir in the oil.

Create a well in the center of the dry ingredients and pour in the yeast mixture. Mix with a spatula or chopsticks until a shaggy dough forms.

Transfer the dough to a lightly floured surface and knead for about 10 to 12 minutes. The dough is ready when it is completely smooth, stretchy, and no longer sticks to your hands.

Shape the dough into a ball and place it back in the bowl. Cover with a damp towel and let it rest in a warm spot for at least 1 hour, until it has doubled in size.

Punch the dough down to release air and move it to a floured surface. Create a hole in the center and stretch the dough into a ring about 10 inches in diameter, then slice into 12 equal pieces.

Keep the dough pieces covered with a damp towel as you work. Roll one piece into an oval, brush the top lightly with oil, and fold it in half. Place the folded bun onto a square of parchment paper inside the bamboo steamer.

Repeat with the remaining dough, leaving space between buns in the steamer to allow for expansion. Cover the steamer and let the buns proof for 15 to 20 minutes.

Fill a wok or large skillet with enough water to touch the bottom edge of the steamer without reaching the buns. Bring the water to a boil over high heat, then reduce to medium and place the steamer on top.

Steam for 8 to 10 minutes. Turn off the heat but leave the lid closed for another 10 minutes—this prevents the buns from collapsing due to a sudden temperature change.

Transfer the buns to a plate and keep them covered with a dry towel until ready to serve to prevent the skins from drying out.