To make Vietnamese cá kho tộ, fish is cooked in a savory-sweet caramel sauce, both seasoning the fish and keeping it moist. To ensure our sauce was dark and savory, not cloying, we cooked the sugar a little longer, creating a darker and slightly bitter caramel. We found that a tablespoon of coconut water helped the sugar caramelize faster, getting us the deep coffee color we were after in just minutes. Keep a close eye on the caramel to prevent it from burning and becoming too bitter. Dark, oily fish are common in this dish, but we liked it better with a firm whitefish. Note that cooking times are for fillets about ¾ inch thick.