Cut chicken into 1-inch-square pieces. In a mixing bowl, combine cornstarch, soy sauce, hoisin sauce, sugar, cilantro, ginger and garlic. Add chicken and toss to coat. Refrigerate, covered, at least 30 minutes.
Heat oven to 400 degrees F. Fold each piece of foil in half twice to make a 6-inch square. Place a spoonful of chicken in the center of each piece of foil, dividing it equally. Fold foil squares on the diagonal; fold edges to seal. Place on a nonstick baking sheet and bake 20 minutes, or until chicken is no longer pink in the center.