In a large bowl, combine the all-purpose flour, instant yeast, granulated sugar, and salt. Mix the dry ingredients until well incorporated.
Gradually add the warm milk while stirring, and then add the vegetable oil. Continue to mix until the dough starts to come together.
Turn the dough out onto a lightly floured surface and knead for about 8-10 minutes, or until it becomes smooth and elastic.
Shape the dough into a ball and place it back into the bowl. Cover the bowl with a damp cloth and let the dough rest for about 1 hour, or until it has doubled in size.
In a bowl, combine the diced char siu pork, hoisin sauce, soy sauce, and sesame oil. Mix well to coat the pork with the flavorful sauces.
If desired, you can add cornstarch to the filling mixture to create a thicker consistency.
Once the dough has doubled in size, punch it down to release the air.
Turn the dough out onto a floured surface and divide it into small portions, each weighing about 40-50 grams.
Take a dough portion and flatten it with your palms to form a small circle, leaving the center slightly thicker than the edges.
Place a spoonful of the char siu pork filling in the center of the dough circle.
Gently gather the edges of the dough and pleat them together, twisting and pinching to seal the bao. Repeat this process for the remaining dough portions and filling.
Place each filled bao on a small piece of parchment paper to prevent sticking during steaming.
Arrange the filled bao on a steamer tray, leaving enough space between them to expand during steaming.
Cover the steamer with a clean cloth or a lid to prevent condensation from dripping onto the bao.
Steam the bao over high heat for about 15-20 minutes, or until they have puffed up and the dough is cooked through.