I love the crunchy white rolls Vietnamese bakeries use for their pork banh mi. If you can't find them,aim to buy the crispest and lightest bread you can for these. Tonkatsu sauce is available in Asian grocery shops;I can't resist drizzling sriracha sauce all over the chicken for a big chilli hit. Pork or white-fleshed fish fillet would also work well when crumbed here,and the chicken itself is great served on steamed rice with the tonkatsu and pickled ginger.