Combine ketchup, soy sauce, white vinegar, brown sugar, and white sugar in a saucepan.

Whisk cold water (or pineapple juice) and cornstarch until smooth.

Whisk the cornstarch mixture into the ketchup mixture and bring to a boil, stirring constantly.

Reduce heat to medium-low and simmer until thickened, then remove from heat and let cool.

In a large bowl, combine flour, cornstarch, baking powder, baking soda, and garlic powder.

Add water and sesame oil and whisk until smooth.

Add chicken pieces and mix until well coated.

Heat oil in a deep-fryer or Dutch oven to 375 degrees.

Carefully dip the battered chicken pieces in hot oil and fry until golden brown (3-4 minutes).

Remove to a paper towel.

Sprinkle with seasoning salt immediately after frying if desired.

Serve immediately with sweet and sour sauce.