In a large mixing bowl, combine all-purpose flour, granulated sugar, instant yeast, baking powder, and salt. Whisk together until well combined.

Gradually add warm water and vegetable oil to the dry ingredients. Mix until a sticky dough forms. Avoid overmixing to prevent dense buns.

Transfer the dough to a floured surface and knead for about 5-7 minutes, or until smooth and elastic. The dough should spring back when poked gently.

Place the kneaded dough in a lightly oiled bowl, cover with a damp cloth, and let it rise in a warm area for about 30 minutes, or until it doubles in size.

While the dough rises, prepare the custard filling by combining milk, granulated sugar, and cornstarch in a saucepan. Stir over medium heat until thickened.

Once thickened, remove from heat and quickly whisk in egg, vanilla extract, and unsalted butter. Stir until smooth. Let the custard cool slightly.

After the dough has risen, punch it down to release air. Divide into 8 equal portions and shape each into a ball. Roll out each ball into a flat circle.

Place custard filling in the center of each circle. Gather the edges and pinch them together to seal, forming a bun.

Prepare your steamer by lining it with parchment paper squares. Place the bao buns in the steamer, leaving space in between. Cover and let them rise again for about 15 minutes.

Bring water to a simmer in the steamer pot. Steam the buns on medium heat for 12-15 minutes until they appear puffed and slightly shiny.

Carefully remove the buns from the steamer and let them cool for a few minutes before serving. Enjoy them warm for the best flavor and texture!