Pat the chicken pieces completely dry with paper towels. In a medium bowl, combine cornstarch, all-purpose flour, ginger-garlic paste, salt, and black pepper. Toss the chicken pieces in the mixture until evenly coated with no dry spots remaining.

Heat about 2 inches of neutral oil in a deep pan over medium-high heat to 350-375°F (a small piece of chicken coating should sizzle immediately when dropped in). Working in batches to avoid overcrowding, fry the chicken pieces for 5-6 minutes until golden brown and crispy. Drain on paper towels and set aside.

Heat 3 tablespoons of cooking oil in a large wok or skillet over high heat until it shimmers. When the oil is ready, add the chopped garlic and green chilies. Stir constantly for about 10 seconds until fragrant but not browned. Watch the garlic closely as it can burn quickly at high heat.

Add the onion cubes and bell pepper cubes to the wok. Stir-fry for 60-90 seconds until slightly softened but still crisp with bright color. Add the dark soy sauce, sweet chili sauce, red chili paste, tomato ketchup, rice vinegar, salt, and black pepper. Stir everything together for 30 seconds to combine and heat through.

In a small bowl, mix the cornstarch with 1 cup of cold water until completely smooth with no lumps. Pour this slurry into the wok while stirring continuously. The sauce will thicken within 1-2 minutes and should coat the back of a spoon. If too thick, add a splash more water.

Add the crispy fried chicken and mango cubes to the wok. Toss gently to coat everything in the glossy sauce. Cook for 1-2 minutes to warm the mango through without making it mushy (it should still hold its shape). Garnish with fresh green onions and serve immediately while the chicken is still crispy.