To the bowl of a stand-up mixer, with the hook attachment, pour in the warm water, milk and active dry yeast. Allow to stand until activated and foamy, about 5 minutes.
In another medium bowl, whisk together the bread flour, sugar, salt, baking powder and baking soda.
To the stand-up mixer, slowly add the bread flour mixture. And then pour in the melted butter. Continue mixing on the lowest speed for 8 minutes. During this time, the flour should pull away from the bowl and become a cohesive dough. Remove the dough from the bowl and shape it into a ball. Return it to the bowl and cover the bowl with a clean towel. Place it in a warm place until it’s doubled in size, about 1 hour.
Punch the dough down and transfer it to a lightly floured countertop. Using a rolling pin (or wine bottle), roll it out so it’s about 1/2-inch thick. Using a 3-inch cookie cutter, stamp out as many bao buns as you can manage.
Fold the rounds over into a moon shape, like a taco. Lightly roll them using the rolling pin, flattening them slightly. Allow them to proof on the counter for about 15 minutes.
Find a saucepan or pot that fits your steam basket. Fill it with a few inches of water and place it over medium-high heat. Add a round of parchment to the bottom to the center of the steam basket (this will prevent the bao buns from sticking!).
Remove the steam basket from the saucepan or pot and add three or four bap buns. Place the top on and transfer it to the saucepan. Cook for about 5 minutes, until the bao buns are puffy and soft to the touch. Remove from the parchment and set aside. Repeat with the remaining bao buns.
Keep in a 200 degree F oven to keep them warm while you cook the rest.