How to Make It
In a medium bowl, combine the water with the sugar and yeast and let stand until foamy, about 5 minutes.
Meanwhile, sift the flour into a large bowl. Using a pastry blender, cut in the shortening. Add the baking powder to the yeast mixture, then stir it into the flour. Scrape the dough out onto a work surface and knead until smooth and elastic. Return the dough to the bowl, cover with plastic wrap and let rise until tripled in bulk, about 2 hours.
Lightly oil a large baking sheet. Punch down the dough and divide it in half. Cover half of the dough with plastic wrap. Roll the other half into a cylinder about 1 inch in diameter. Cut the cylinder into 8 equal pieces and roll each piece into a ball. Using a rolling pin, roll each ball into a 4-inch round about 1/8 inch thick. Brush the rounds with sesame oil, fold in half and transfer to the baking sheet. Repeat with the remaining dough. Cover the baking sheet loosely with plastic wrap and let the buns rise until doubled in bulk, about 1 hour.
Generously oil the bottom of a triple-tiered bamboo steamer. Bring 2 inches of water to a boil in a wok. Arrange the buns in the steamer without crowding, set over the boiling water and steam until puffy and cooked, about 5 minutes. Alternatively, oil the basket of a single-layer metal steamer and steam the buns in batches. Immediately transfer the buns to a platter and cover with a tea towel. Serve hot. Let your guests split the buns and fill them with the meat and scallions. Pass the hoisin sauce separately.