1. To prepare the filling, rub five-spice powder evenly over pork. Heat a grill pan over medium-high heat. Coat the pan with cooking spray. Add pork to pan; cook 18 minutes or until a thermometer registers 155°, turning pork occasionally. Remove pork from pan, and let stand 15 minutes.
2. Cut pork crosswise into thin slices; cut slices into thin strips. Place pork in a medium bowl. Add onions and next 7 ingredients (through 1/4 teaspoon salt); stir well to combine. Cover and refrigerate.
3. To prepare dough, combine 1 cup warm water, sugar, and yeast in a large bowl; let stand 5 minutes.
4. Lightly spoon flour into dry measuring cups; level with a knife. Add flour, oil, and 1/4 teaspoon salt to yeast mixture; stir until a soft dough forms. Turn dough out onto a lightly floured surface. Knead until smooth and elastic (about 10 minutes). Place dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place (85°), free from drafts, 1 hour or until doubled in size. (Gently press two fingers into dough. If indentation remains, dough has risen enough.)
5. Punch dough down; let rest 5 minutes. Turn dough out onto a clean surface; knead in baking powder. Let dough rest 5 minutes.
6. Divide dough into 10 equal portions, forming each into a ball. Working with one dough ball at a time (cover remaining dough balls to keep from drying), roll ball into a 5-inch circle. Place 1/4 cup filling in center of dough circle. Bring up sides to cover filling and meet on top. Pinch and seal closed with a twist. Repeat procedure with remaining dough balls and filling.
7. Arrange 5 buns seam side down, 1 inch apart, in each tier of a 2-tiered bamboo steamer. Stack tiers; cover with lid.
8. Add water to a large skillet to a depth of 1 inch; bring to a boil over medium-high heat. Place steamer in pan; steam 15 minutes or until puffed and set. Cool 10 minutes before serving.