Fry The Chicken:

Wash the chicken and pat it dry using paper tissues.
Cut into small bite-size cubes.
Add cornstarch, all-purpose flour, ginger garlic paste, salt, and black pepper powder to a medium-sized mixing bowl and mix well.
Add the cubed chicken pieces to the bowl and coat them nicely with the flour mixture.
Heat oil for frying in a pan over high heat.
Once the oil is hot, drop half of the chicken pieces in the hot oil and fry until crisp and golden brown from the outside.
Drain and keep aside.
Fry the remaining chicken. Drain and keep aside.

Make The Sauce:

Heat oil in a wok over high heat.
Once the oil is hot, add garlic and green chilies and fry for 3-4 seconds.
Add onion and bell peppers and cook for a minute.
Now add dark soy sauce, sweet chili sauce, red chili paste, tomato ketchup, rice vinegar, salt, and black pepper powder to the wok and mix well.
Mix cornstarch in water to make a slurry and add the slurry to the wok.
Cook for a minute or till the sauce thickens and the cornstarch is cooked. Add more water if required.
Add the fried chicken and mango pieces to the wok and mix well.
Garnish with chopped green onions and serve immediately.