In a small bowl, mix together the ingredients for the Tonkatsu sauce. Cover with plastic wrap and refrigerate until serving.
Preheat oven to 425 degrees. Place an oven-safe baking rack on a rimmed baking sheet. Lightly spray with non-stick spray.
Trim the chicken of any unwanted fat. If using chicken breasts, cut them crosswise to form 4 chicken cutlets. Lay cutlets between two layers of plastic wrap. Pound out chicken until ¼-inch thick. Season with ½ teaspoon salt and pepper on both sides of the chicken.
Combine flour and ½ teaspoon salt and garlic powder in a shallow dish and mix well. In a separate shallow dish, whisk eggs together with ½ teaspoon salt and water. Place the breadcrumbs in a final shallow dish.
Working with one cutlet at a time, dredge the seasoned chicken in flour to coat, shaking off excess. Dip in egg, coating both sides, let excess drip off, then press into bread crumbs, using your hands to ensure the chicken cutlet is evenly coated. Place breaded chicken onto the prepared baking sheet and repeat until the cutlets are all breaded.
Spray the chicken generously with cooking spray. It should be enough to make the breadcrumbs glisten.
Bake for 15-18 minutes, until golden and the chicken has reached 165 degrees Fahrenheit.
Slice cutlets into thick slices and serve with the Tonkatsu sauce over rice, with a side of shredded cabbage if desired.
Storage: Leftover Tonkatsu sauce can be stored in the refrigerator for up to 7 days in an airtight container. For the crispiest of chicken, Chicken Katsu is best served immediately after preparation. However, leftovers can be stored in an airtight container in the refrigerator for up to 3 days. Reheat by broiling under high heat for 1-2 minutes per side or heating in a dry skillet. Do not microwave or the breading will become soggy.