Step 1
Stir together yeast and water in a small bowl; let stand 5 to 10 minutes. Whisk in 2 tablespoons of the oil.
Step 2
Pulse flour sugar baking powder and salt in a food processor a few times to combine. With machine running pour yeast mixture through feed tube. Start slowly then pour faster. Process until a dough ball forms sides of bowl are nearly clean and dough is medium-soft and tacky but doesn't stick to fingers.
Step 3
Turn dough ball and bits out onto an unfloured counter and gather into a neat ball. If dough feels stiff wet hands and knead in the water from your hands. Transfer to an oiled bowl. Cover with plastic wrap and let rise in a warm place until nearly doubled about 45 minutes.
Step 4
Meanwhile cut twenty-four 3-inch squares of parchment paper. Put remaining 1 tablespoon oil in a small bowl.
Step 5
Cut dough in half. Roll one half into a rope about 1 1/2 inches thick and 14 inches long; cut crosswise into 12 equal pieces. Roll each piece into a ball then smack it with your palm into a disk about 1/3 inch thick. With a rolling pin roll each disk into a thin oval about 2 1/2 inches wide by 4 1/2 inches long. Brush half of each oval with oil then fold in half to form buns.
Step 6
Set each bun on a parchment square; transfer to a bamboo or metal steamer tray spacing about 3/4 inch apart and away from steamer walls. Set any that don't fit in the steamer on a baking sheet. Loosely cover with a kitchen towel. Let rise in a warm spot until about 1 1/2 times the original thickness 20 to 30 minutes.
Step 7
Meanwhile fill a steamer pan or a pot halfway with water; bring to a rolling boil over high heat.
Step 8
Steam buns over boiling water 2 trays at a time covered with lid until puffy and dry-looking about 8 minutes.