Add the flour into the bowl of a stand mixer fitted with a dough hook, or into a large mixing bowl. Whisk the sugar, baking powder, kosher salt, and instant yeast into the flour until evenly distributed. In a separate bowl, whisk together the warm water and buttermilk or Greek yogurt. Pour into the dry mixture and mix on low speed (or with a sturdy wooden spoon) until a shaggy dough forms. If using the stand mixer, bump the speed up one level and knead with the dough hook until a smooth dough forms, about 5 minutes. If mixing by hand, turn onto a lightly floured surface and knead until a smooth dough forms. If kneading by stand mixer, you can simply remove the dough hook and cover with a towel. If kneading by hand, lightly flour a clean bowl and place the dough in the bowl before covering with a towel. Either way, let rest in a warm, draft-free place for 30 to 45 minutes.
While the dough rests, assemble a wok or frying pan with a stacked steamer or place a stainless steel vegetable steamer in a pot that has walls that are at least 5-inches high. Whichever pot you use, you want about an inch of water in the bottom, or enough to boil for a while without bubbling up through the bottom of the steamer. You need some space between the water and your bao buns.
Liberally flour a counter or dough mat. Turn the dough onto the mat and dust the top of the dough with more flour. Place your rolling pin in the center of the dough, and roll away from yourself. Lift the rolling pin, bring back to the center and roll toward yourself. Turn the rolling pin perpendicular to yourself, place in the center and roll toward the right. Lift the rolling pin, replace in the center and roll toward the left. Take a moment to shimmy the dough around to be sure it isn't sticking, then repeat the rolling instructions above until the dough is 1/2-inch thick evenly. Dip a round cookie cutter in extra flour and press straight down into the dough. When you reach the bottom, give it a little twist to be sure it has cut cleanly. Lift the cutter, dip in the excess flour and repeat, making the circles as close together as possible to leave as little unused dough between the rounds as you can. When you have cut as many circles as possible, lift the extra dough away, press together, and return the extra dough to the bowl, covering again with a towel and letting rest 30 minutes before re-rolling and cutting more circles from it.
Use the back of a spoon or pastry brush dipped in canola or sunflower oil to coat half of each round. Gently fold the un-coated half over the coated half of the round of dough.
Place a rectangle of parchment coated with non-stick cooking spray in the steamer. Gently ease the folded buns onto the parchment, leaving 1/2-inch of open space around the perimeter of each bun. Place the lid on the steamer (or leave the pot lid slightly askew to allow the steam to vent) and let the buns steam for 6 minutes. Carefully transfer the finished Bao Buns to a plate and repeat with the remaining bun semi-circles.
Fill the cooked Bao Buns as desired and serve within an hour of being steamed, but preferably while they are still warm.