To prepare the dough: Place water, sugar, and yeast in the bowl of an electric mixer. Let sit for 5 minutes.
Spoon flour into measuring cup and level with a knife. Add flour, oil, and salt to the yeast mixture. Stir with a wooden spoon just until it comes together a little, you will know to stop because the dough will start to stick to the spoon. Attach the dough hook to your mixer and knead for a few minutes, until the dough comes together and is smooth and elastic. (You can also do this by hand, kneading for about 10 minutes.) Spray a large bowl with cooking spray and place the dough inside. Spray the top of the dough with a light spray of cooking spray and cover with plastic wrap. Let sit in a warm environment until it doubles in size, at least one hour.
While the dough is rising, make the filling. Heat the oil in a large nonstick skillet over medium-high heat. Add the scallions, the bell pepper, and ginger and cook for about 1 minute.
Add the turkey and chili powder, breaking it up with a wooden spoon, until browned and cooked through, about 5 minutes. Stir in the soy and teriyaki sauce. Let cook for an additional 1 minute, then remove from heat. Place in a bowl and let cool until you are ready to fill your buns.
Once dough has risen, punch it down and let it sit 5 minutes. Turn it out onto a clean work surface (I like to use a large cutting board or my Roul’Pat) and sprinkle it with the baking powder. Knead in the baking powder (will take just a minute or two) and form a large ball. Let it sit for 5 minutes.
Cut dough into 10 sections. Flatten each section into a large (5-inch) circle. Place about 2-3 tablespoons of filling in the center of each circle. Bring the sides up to cover the filling and pinch the seams at the top, twisting slightly to seal closed. Repeat with all 10 circles.
If you have a 2-tiered bamboo steamer, arrange the buns 1 inch apart, 5 on each tier. Stack the tiers and cover with the lid. Place steamer in a large skillet that has 1-inch of boiling water in the bottom. (Continue to step 8.)You do not need a steamer; I don’t have one.
If you don’t have a bamboo steamer, do what I do: use a large pasta pot, one with a colander insert. Place about 1-2” of water in the bottom of the pot (you do not want it to touch the holes of the colander) and boil over medium heat. Spray the colander with cooking spray. Working it batches, place 5 buns on the bottom of the colander, 1-inch apart.
Steam for 10-15 minutes (mine took 12) until they are puffed. Remove from steamer to a cookie sheet lined with waxed or parchment paper. Repeat with remaining buns if necessary (spraying colander again with cooking spray). Let cool 10 minutes before eating.