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1
Stir the warm water, yeast and sugar together in a large bowl. Let stand until the mixture begins to bubble, about 10 minutes. Meanwhile, cut fifteen 3-inch squares of parchment paper and set them aside.
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2
Gradually stir in the all-purpose and cake flours, then beat the dough with a wooden spoon or heavy duty mixer until it holds together.
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3
Turn the dough out onto a lightly floured board and knead, adding more flour as necessary to prevent sticking, until smooth and elastic, 5 to 10 minutes. Using the vegetable oil, lightly grease the inside of a 2-gallon resealable plastic bag. Slip the dough into the bag, add the 1/2 teaspoon water and seal the bag. Let rise in a warm location until the dough doubles in size, 2 to 3 hours.
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4
Remove the dough from the bag and place on a lightly floured surface. Sprinkle the baking powder over the dough and knead, adding flour as necessary to prevent sticking, until the dough is smooth, about 5 minutes.
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5
Divide the dough into 15 equal pieces. Roll one piece into a round about 4 inches in diameter. Press the edges of the dough circle with your fingers to make them slightly thinner than the center. Place about 2 tablespoons of the filling in the center of the round. Gather the edges of the dough over the filling, completely enclosing the filling, and twist the edges together to seal.
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6
Place the bun, twisted-side up, on a parchment paper square and set on a baking sheet. Repeat with the remaining dough and filling. Cover with a kitchen towel and let the buns rise in a warm location until puffy, about 30 minutes.
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7
Prepare a wok for steaming. Place half the buns in the steaming basket, cover the basket and steam over high heat until the dough is cooked and the buns have slightly cracked on top, 15 to 20 minutes. Repeat with the remaining buns. Serve warm.
