Cut slits along the fat (this prevents pork from curling as it fries). Tenderize pork until about 1 cm thick.
Evenly season pork with salt, black pepper and garlic powder on both sides.
Place flour in a large shallow bowl, beaten eggs in a medium bowl and panko in a third bowl. Dredge each pork loin first in flour, then in egg and lastly panko - ensuring no bald spots.
In a large wok or heavy pot, add oil and heat over medium heat. Once oil is hot (check by inserting a wooden chopstick and look for bubbles to form), carefully lower breaded pork away from you to prevent oil splash back.
Deep frying on each side until golden brown, 2 minutes per side and until the pork reaches and internal temperature of 145 F or juices run clear. Remove pork with a slotted spoon and rest the pork on a wire rack or large plate or baking sheet lined with paper towels to excess drip the oil. The residual oil will make the crispy pork soggy.
Optional: To take your sandwich to the next level, in a bowl, combine grated cabbage, salt, rice vinegar and Japanese mayo. Mix well. Or simply substitute with plain shredded cabbage.
Optional: if you're making homemade tonkatsu sauce, combine Sauce as listed above in a bowl and whisk together. If you're using store bought, please skip this step.
Butter both slices of bread. Layer a piece of pork katsu on top, drizzle store bought or homemade Tonkatsu sauce over top, with an equal portion of cabbage slaw (optional) and close off with slice of bread.
Optional: Slice off the crust and slice sandwich in half. Enjoy!