How to Make It
Whisk together egg white, white pepper, 1 tablespoon of the cornstarch, and 1 teaspoon of the soy sauce in a large bowl. Add catfish pieces, and gently mix until fish is evenly coated. Let stand 15 minutes.
Whisk together water, hoisin sauce, black vinegar, sugar, salt, and remaining 3 tablespoons soy sauce and 2 tablespoons cornstarch in a large bowl; set aside.
Heat 4 cups of the oil in a small Dutch oven over medium-high to 300°F. Carefully add half of the catfish pieces to hot oil; cook, occasionally gently moving fish, until golden brown, 1 to 2 minutes per side. Transfer fish to a plate lined with paper towels to drain. Repeat with remaining catfish.
Heat remaining 2 tablespoons oil in a wok over high until lightly smoking. Add scallions, dried chiles, garlic, and black pepper. Cook, stirring constantly, until fragrant and garlic is golden, about 30 seconds. Stir in hoisin sauce mixture. (The sauce should thicken almost immediately.) Carefully add fried catfish, and stir gently to coat. Transfer mixture to a plate. Sprinkle with peanuts, and serve immediately.