Combine the flour, sugar, salt and yeast in a mixing bowl.
In another small bowl, add the milk, water and butter, stirring until everything is well combined.
Stir the liquids into the flour mixture until the dough starts to come together, then turn the dough out onto a floured countertop and knead for about 10 minutes. (Or use a stand mixer with a dough hook.)
Spray a bowl with cooking spray and place the dough in it, covering with plastic wrap or a damp towel, letting it rise for about 90 minutes or until the dough doubles in size.
Once it is doubled, dump it out onto a floured countertop again, knead it briefly, and split it into 20 even balls.
Cut some parchment paper into rectangles—about 2.5 by 3.5 inches—one for each ball, and roll the balls out into an oval shape about the same dimensions as the piece of parchment.
Once the dough is rolled out, brush each oval with the olive oil.
Place a chopstick or bamboo skewer in the middle of each oval (this will leave a little bit of an empty spot in the dough when you slide it out) and fold it over.
Leaving the dough on the parchment, position them into the trays of a bamboo steamer, covering each section with plastic wrap or a damp towel, and letting them rise for an hour.
To steam the buns, boil a few inches of water in a skillet that is large enough to fit your bamboo steamer. You want just enough water that it will come up on the sides of the steamer but not actually touch the buns in the bottom section.
Once the water is boiling, place the steamer basket in the pan and steam the buns for 10 minutes.
Carefully remove the steamer basket and serve the bao immediately with your favorite fillings.