In a medium sized bowl, stir together garlic, ginger, soy sauce, sugar, and sesame oil. Place cornstarch in a small bowl and very slowly stir about 1-2 tbsp of the liquid mixture into the cornstarch until the mixture is smooth. Then, stir the cornstarch mixture back into the larger bowl. Once well mixed, add in chicken. Stir so that chicken is evenly covered and let chicken sit in marinade for 30 minutes.