Start by cutting the boneless skinless chicken breasts into small, bite-sized cubes. Aim for uniformity in size so that they cook evenly. Place the cubed chicken in a large mixing bowl.
Pour the buttermilk over the chicken cubes, ensuring they are well-coated. Cover the bowl and let it marinate in the refrigerator for at least 30 minutes.
In a shallow dish, combine the all-purpose flour, garlic salt, and a pinch of pepper. Mix well to ensure the seasonings are evenly distributed.
After marinating, remove the chicken from the refrigerator. One piece at a time, take the marinated chicken and dredge it in the flour mixture, ensuring each piece is fully coated. Shake off any excess flour.
Next, roll each floured chicken piece in the panko crumbs, pressing gently to help the crumbs adhere.
In a frying pan or deep fryer, heat vegetable oil over medium-high heat. You want enough oil to submerge the chicken pieces, about 2-3 inches deep.
Carefully add the coated chicken pieces to the hot oil in batches, making sure not to overcrowd the pan. Fry for about 4-6 minutes or until golden brown and cooked through, turning occasionally for even cooking.
Once cooked, use a slotted spoon to remove the popcorn chicken from the oil and place it on a plate lined with paper towels to drain the excess oil. Serve immediately with your favorite dipping sauce!