Make the Brine: In a medium bowl, whisk together the water, sugar, sesame oil, onion powder, 1/2 teaspoon salt, and garlic powder.
In an 8x8-inch baking dish, arrange the tofu cubes in a single layer. Pour the brine over the tofu. Set aside to marinate for 1 hour.
Make the Coating: In a medium bowl, whisk together the flour, cornstarch, 3/4 teaspoon salt, white pepper, and black pepper.
Heat a large skillet over medium-high heat and add the oil.
Dredge the tofu pieces in the flour mixture, then carefully place the coated tofu into the hot oil. Cook for 3-4 minutes per side or until golden brown on all sides. Using a slotted spoon, transfer the cooked tofu to a plate lined with paper towels to absorb any excess oil.
Transfer to a serving plate, sprinkle with scallions, and serve warm.