Cut the chicken into 1-inch pieces. Place the cornstarch in a gallon zip-top bag, add the chicken, seal, and shake well until all pieces are evenly coated.
In a medium bowl, whisk together the hoisin sauce, soy sauce, brown sugar, minced garlic, rice vinegar, sesame oil, ground ginger, and crushed red pepper flakes. Set aside.
Cook the ramen noodles according to package directions, discarding the seasoning packets. Drain and set aside.
Heat the vegetable oil in a large skillet over medium-high heat. Shake off excess cornstarch from each chicken piece and add to the hot skillet. Cook, turning occasionally, until golden brown on all sides and cooked through. Work in batches as needed to avoid crowding the pan. Transfer the cooked chicken to a plate.
Pour the prepared sauce into the same skillet. Bring to a boil, then reduce heat and simmer for about 5 minutes until the sauce is thickened and glossy. Adjust heat and red pepper to taste.
Return the chicken to the skillet and toss to coat in the sauce. Add the cooked ramen noodles and toss to combine. Add extra soy sauce if desired. Garnish with sliced green onions and sesame seeds. Serve immediately.