Whisk together the cornstarch, flour, baking powder, egg, seltzer water, and 1 tablespoon of oil in a large bowl. Set aside.
Season the chicken cubes with salt and pepper.
Transfer the chicken cubes to the flour-water batter and mix them together so the chicken cubes are fully coated. Set aside.
Prepare a cookie sheet lined with a few layers of paper towel for later.
Heat a 6-qt Dutch oven or large pot with oil, so it comes up at least 1/2” in the pot, over medium-high heat. Use an instant read or candy thermometer to check that the oil is between 350°F and 375°F.
Carefully lower 1 chicken cube at a time, using a slotted spoon, into the hot oil, only adding 1⁄3 of the chicken pieces in this batch. Cook chicken for about 4 minutes in the hot oil, stirring occasionally, and carefully flip over each piece halfway through. Place cooked chicken cubes onto the prepared paper towel-lined baking sheet. Repeat in two more batches, so as not to overcrowd the pot.
Transfer the fried chicken to a large bowl & pour the sauce over it. Toss everything together.
Serve with rice and chopped scallions!