||Whisk together fish sauce and sugar in a bowl until the sugar is completely dissolved. Stir in 1 tablespoon vegetable oil and white pepper to taste.
||Place the fish in a food processor and pulse to make a fine paste, about 1 minute. Add the fish sauce mixture, potato starch, and baking powder and continue pulsing until the mixture has a firm, paste-like consistency, about 1 minute more.
||Bring a pot filled with water to a boil over high heat. Pinch off about 1 teaspoon of the fish paste and form it into a ball. Repeat the process, dropping each ball into the boiling water to cook. Once the balls float to the surface, cook for 1 minute more and remove with a slotted spoon. Drain and cool completely. At this point you can use fish balls in hu tieu do bien, seafood noodle soup, or freeze them.
||Heat the remaining vegetable oil in a nonstick pan over medium-high heat. Pan-fry the fish balls, shaking the pan occasionally, until golden on all sides, 3 to 5 minutes. Serve with spicy lemongrass soy sauce for dipping.