Preheat the oven to 425 degrees F. Line a baking sheet with foil and lightly grease with oil.
Combine the cauliflower and 3 tablespoons of the oil in a large resealable plastic bag. Seal and shake until well coated. Add 1/2 cup of the cornstarch, 1 teaspoon salt and the pepper, then reseal and shake until well coated.
Heat the remaining 1/3 cup oil in a large skillet over medium-high heat. Once hot, add the cauliflower and fry, tossing to cook on all sides, until deep golden brown in spots, about 10 minutes. Transfer the fried cauliflower to the prepared baking sheet and bake until the cauliflower starts to get crispy, 10 to 15 minutes
While the cauliflower bakes, prepare the sauce. Whisk together the sugar, vinegar, soy sauce, ketchup, garlic powder and the remaining 1/2 teaspoon salt in a medium saucepan. Place over medium-high heat and bring to a boil; the sauce should thicken slightly.
Whisk together the remaining 1 tablespoon cornstarch with 2 tablespoons cold water in a small bowl until fully dissolved. Add the cornstarch slurry to the boiling sauce, reduce the heat to low and cook until thickened, about 30 seconds.
Transfer the cauliflower to a large bowl and add half of the sauce. Toss until the cauliflower is well coated, then add the remaining sauce and toss. Garnish with the scallions and sesame seeds and serve with rice.
Cooked White Rice:
Place the rice and 2 1/4 cups cold water in a medium saucepan. Put the pan over medium-high heat. Bring to a boil, reduce to a simmer, cover and cook until the rice is tender, 18 to 20 minutes. Remove the lid and fluff the rice before serving.