Spread the tofu cubes in a single layer over several layers of paper towels, then top with more paper towels and press gently to remove excess moisture. Season the tofu generously with salt and pepper, then toss in a large bowl with the larger portion of cornstarch until every cube is evenly coated.

Heat 4 tablespoons of the oil in a 12-inch nonstick skillet over medium-high heat until it shimmers. Add the tofu and cook, turning occasionally, until it is crisp and golden brown on all sides, about 8 to 10 minutes; transfer the fried tofu to a plate lined with paper towels.

Wipe out the skillet and add the remaining oil, sugar, and 1/4 cup of the water. Cook over medium-high heat, without stirring, until the mixture turns a dark amber color and smells slightly toasted, about 5 to 7 minutes.

Carefully and slowly pour the remaining 1 1/2 cups of water into the skillet—the caramel will steam and harden initially, but it will melt back down as it heats. Stir in the onion, garlic, fish sauce substitute, and a generous pinch of pepper, and simmer until the onion has softened and the liquid has reduced slightly, about 5 minutes.

Whisk the remaining 2 teaspoons of cornstarch with a tablespoon of water in a small bowl to create a slurry. Whisk this into the sauce and simmer for 1 minute until thickened and glossy.

Add the fried tofu back into the skillet and toss gently to coat every piece in the sauce. Serve immediately over rice, topped with the cilantro, chopped peanuts, and scallions for a fresh, crunchy finish.