Set a wire rack over a rimmed baking sheet and cover half with a double layer of paper towels.
Pat the chicken breasts dry with paper towels and cut the chicken into Place the chicken cubes on a paper towel-lined plate, pat dry with another paper towel and season with kosher salt and freshly ground black pepper.
Pour 2 quarts oil (or enough to measure about 1½-inches deep) in a large Dutch oven set over medium-high heat. Bring the oil to 350 degrees.
While the oil heats, whisk together flour, cornstarch, baking powder, baking soda, 2 teaspoons kosher salt and 1 teaspoon black pepper in a large mixing bowl. Add the water and the remaining 3 tablespoons oil; whisk until smooth.
Place the chicken cubes in the batter and stir to completely coat.
Once the oil reaches 350°F, use a fork to carefully drop each battered chicken cube into the hot oil. (Have a second fork ready to use to release the chicken into the oil, if necessary.)
Check the oil temperature frequently and adjust the heat, up or down, to keep the temperature between 350-375 degrees F.
Use a spider skimmer to stir the chicken pieces gently to prevent them from sticking together. Cook until chicken is golden, 2-3 minutes each batch. Transfer chicken to paper towel-lined half of the prepared rack.
Repeat process until all the battered chicken is fried.
When all the chicken has been cooked, remove the pan from the heat and, very carefully, pour all, except 1-2 tablespoons oil, into a heatproof bowl.
Return the Dutch oven to the stovetop set over medium heat. Once the oil is hot, add the red pepper, half the cut scallions and all the pineapple chunks. Cook 2-3 minutes or until the red pepper is crisp-tender.
Add 1-cup sauce to the vegetable mixture and cook until sauce is heated through, about 1 minute.
Add the chicken and stir gently to coat the chicken. Cook just enough to heat the chicken through and serve immediately over cooked white rice and with the remaining sweet and sour sauce.
Garnish with the remaining half of cut scallions.