Daigaku Imo (Japanese Candied Sweet Potatoes)

Method

1. 1Scrub the sweet potatoes under running water, leave the skin on, and cut them into irregular chunks about 3 cm across (ran-giri): cut off a piece at a slant, roll the potato a quarter turn, cut again. The angled faces give you more surface to brown and to hold the syrup. Drop the pieces into a bowl of cold water as you cut and leave them 10 minutes, which pulls out surface starch and some of the bitterness.
2. 2Drain the pieces in a colander, then pat each one dry with a clean kitchen towel. Water left on the surface makes the oil spit and keeps the potato from browning, so this step matters more than it looks.
3. 3Pour the oil into a 24 cm frying pan. That is only a few millimetres of oil (150 ml spread over a 24 cm circle is about 0.3 cm), and it is enough, because the pieces are turned every 2 minutes and every face gets its time in the oil. Warm it over medium-low heat until a dry wooden chopstick dipped in it releases small, steady bubbles (around 160°C). Lay the pieces in one layer with space between them and cook 8 to 10 minutes, turning every 2 minutes. Low heat is the point: on a hotter flame the outside burns long before the middle softens. They are done when a skewer slides in with no resistance and the cut faces are pale gold.
4. 4Turn the heat to medium-high and fry another 1 to 2 minutes, turning, until the skin is crisp and a deeper golden. Lift the pieces onto a plate lined with paper towel and let the excess oil drain while you make the syrup. Do not leave them long, because the syrup clings best to hot potato.
5. 5Put the sugar, water, mirin and soy sauce into a small, clean pan and heat over medium without stirring, only swirling the pan now and then. After 2 to 3 minutes it boils in large, slow bubbles and drips off a spoon in a thick string. Stop there. Stirring invites white sugar crystals, and a few minutes more turns the syrup into hard candy that glues the pieces together.
6. 6Switch off the heat, tip the hot potato pieces into the syrup and toss with a spatula for about 30 seconds until every side is glossy. Sprinkle over the sesame seeds and toss once more. Cold potato does not take the syrup evenly, so do this straight away.
7. 7Spread the pieces on a tray lined with parchment paper, apart from each other, and leave them 5 minutes. The syrup cools into a thin, slightly crackly shell. Serve warm or at room temperature; pieces piled hot on a plate stick together into one lump.