Cut the chicken breasts into squares (about ½" by ½" thick).
In a medium size bowl, place the chicken chunks, cornstarch, salt and black pepper; mix until well coated.
In a skillet/pan; heat the oil to medium-high; 350°F (176°C). Wait for at least 3-5 minutes before placng the chicken in the skillet/pan.
Place the coated chicken into the oil and cook for 5-7 minutes or until golden brown. Don't overcrowd the chicken; space them out. Cook in 3 separate batches.
Remove the chicken from the skillet/pan and place on a baking rack or a plate with paper towel (to drain the oil).