In a small bowl stir together the yeast and sugar with ¾ cup lukewarm water and let sit for 10 minutes.
Add the yeast mixture, flour, salt, baking powder, baking soda, and fat (or shortening) to the bowl of a stand mixer fitted with a dough hook attachment. Mix on the lowest speed until the dough gathers in a ball on the hook, about 8 minutes. Lightly oil a large bowl and add the dough, turning to coat it with the oil then cover with plastic wrap and place in warm place to let rise until doubled in size, about 1 hour.
Punch the dough down and turn it out onto a clean work surface. Divide the dough in half then cut each half into 8 equal pieces. Roll each piece into a ball, making 16 pieces, each about the size of a ping-pong ball. Set the balls on a parchment paper lined baking sheet and cover with plastic wrap. Let rise for 30 minutes.
Working one at a time, use a rolling pin to roll a dough ball into a 4" (10cm) long oval. Lightly brush with the oil and fold each oval in half horizontally to form a bun. Place the bun on one of the squares of parchment paper.
Once again, cover with the plastic wrap. Repeat with the remaining buns and let rise for an additional 30 minutes.
Set up a steamer on top of the stove with water on medium-low heat. Work in batches as to not crowd the steamer, steam the buns on the parchment paper for about 10 minutes. Remove the parchment paper and set the buns aside. Buns can also be cooled then frozen in plastic ziplock bags. Reheat in a steamer until completely soft, about 2 minutes.
For the pork filling:
Heat a wok or large sauté pan over medium heat. Add the ginger, garlic and the whites of the green onions and sauté for about 30 seconds. Add the pork and season with salt. Cook, stirring occasionally and using a wooden spoon to break apart the pork, until the pork is browned about 6 minutes.
In a small bowl stir together the soy sauce, hoisin and sriracha, Add the sauce mixture and cook for an additional minute. Use a slotted spoon to remove the pork to a medium mixing bowl. Stir in the remaining green onions and set aside.
For serving:
Spoon the pork filling into the bao buns and garnish with the cucumber, green onions and cilantro. Serve immediately.
Notes
For the fluffiest bao buns, make sure to let the dough rise in a warm, draft-free place. If your kitchen is cool, place the covered dough in the oven with just the oven light on to create a warm proofing environment.