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1
Stir together yeast and water in a small bowl; let stand 5 to 10 minutes. Whisk in 2 tablespoons of the oil.
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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.
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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.
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4
Meanwhile, cut twenty-four 3-inch squares of parchment paper. Put remaining 1 tablespoon oil in a small bowl.
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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.
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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.
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7
Meanwhile, fill a steamer pan or a pot halfway with water; bring to a rolling boil over high heat.
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8
Steam buns over boiling water, 2 trays at a time, covered with lid, until puffy and dry-looking, about 8 minutes.
