Spread the tofu over a paper towel-lined baking sheet and let it drain for 20 minutes to remove excess moisture. Gently press the cubes dry with additional paper towels, then season with salt and pepper.

Pour the first portion of water into a medium saucepan and sprinkle the sugar evenly over the top. Cook over medium heat, gently swirling the pan occasionally—do not stir—until the sugar melts and the mixture turns the color of dark maple syrup, about 7 to 10 minutes.

Stir in the oil and garlic and cook until fragrant. Remove the pan from the heat and slowly whisk in the remaining water; the sauce will sizzle and steam—it's just doing its job. Stir in the onion, fish sauce, the smaller portion of cornstarch, and the pepper.

Return the pan to medium-low heat and simmer vigorously until the onions have softened and the sauce has thickened to a syrupy consistency, 10 to 15 minutes. Remove from the heat and cover to keep warm.

Spread the coating cornstarch in a shallow dish. Working with several tofu pieces at a time, coat them thoroughly in the starch, pressing gently so it adheres, then transfer to a clean plate.

Heat the remaining oil in a 12-inch nonstick skillet over high heat until the oil just begins to smoke. Add the tofu and cook, turning as needed, until all sides are crisp and well-browned, about 10 to 15 minutes.

Transfer the tofu to a paper towel-lined plate to drain briefly, then arrange on a serving platter. Drizzle with the warm caramel sauce and sprinkle with cilantro, peanuts, and scallions. Serve immediately while the tofu is still hot and crackling.