Preheat the oven to 450°F (232°C). Line a baking sheet with foil and lightly grease it.
Butterfly the chicken breast by slicing horizontally through the center to create two thin even pieces; lightly season both sides with salt.
Set up an assembly line with three shallow bowls: flour in the first, beaten egg in the second, and panko in the third. Place the prepared baking sheet at the end.
Dredge each chicken piece in flour, shaking off excess, then dip in the beaten egg, and press into the panko to coat evenly. Transfer to the baking sheet.
Lightly spray the tops of the breaded chicken with cooking spray to help browning. Bake 15 to 20 minutes, until an instant-read thermometer inserted into the thickest part reads 160°F (71°C).
While the chicken bakes, whisk the soy sauce and cornstarch in a very small bowl until smooth. Combine that slurry with the granulated sugar, brown sugar, white vinegar, pineapple juice, and ketchup in a small saucepan.
Bring the sauce to a simmer over medium-high heat, reduce heat if it starts to boil, and simmer 3 to 6 minutes until thickened. Remove from heat.
Remove the chicken from the oven and let rest 5 minutes, then slice. Serve the sliced katsu over white steamed rice with pineapple chunks if desired and sauce on the side for dipping.