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-inch 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.