Peel the sweet potatoes and cut them into rough 1 inch chunks. Rinse in cold water and pat completely dry.

Heat the frying oil to 325°F. Fry the chunks in two batches for 6 to 8 minutes, until a chopstick passes through easily and the outsides are pale gold.

Raise the oil to 375°F and fry each batch a second time for 1 to 2 minutes to firm the exterior. Drain on a wire rack and keep warm.

Warm a serving plate under hot water, dry it, and wipe a thin film of oil across the surface. Set a large bowl of ice water on the table.

Wipe the wok clean or use a small saucepan. Combine the sugar, water, and 1 tablespoon oil over medium heat. Stir gently only until the sugar dissolves, then stop stirring completely.

Let the syrup bubble undisturbed. When the bubbles turn small and tight, the surface calms, and the color reaches the palest amber, around 300°F to 310°F, it is ready. A chopstick lifted from the pan should draw a fine thread.

Remove from the heat immediately. Add all the fried sweet potato at once and fold for 10 to 15 seconds until every piece is coated and glossy.

Turn out onto the oiled plate, separate the pieces slightly, scatter the sesame seeds over while the syrup is wet, and serve at once with the ice water. Each person dips a piece for about one second before eating.