Daigaku Imo (Japanese Candied Sweet Potatoes)

Almost always the heat was too high. In shallow oil the surface browns within a few minutes, while a 3 cm chunk needs 8 to 10 minutes at around 160°C to soften all the way through. Keep the flame at medium-low, turn the pieces every 2 minutes and test with a skewer: it should slide in with no resistance before you raise the heat for the last 1 to 2 minutes. If the outside is already dark and the middle is still firm, cover the pan and cook 3 minutes on low heat. Chunks wider than about 4 cm cause the same problem, so cut them smaller next time.

The syrup went past the right point. Sugar, water, mirin and soy sauce need only 2 to 3 minutes over medium heat; when the bubbles are large and slow and the syrup drips off a spoon in a thick string, switch the heat off. Boil it longer and it sets into candy; stir it while it boils and white sugar crystals appear. If it has already gone hard on the pieces, add 1 tablespoon of water to the pan, warm it for 20 seconds and toss again. If the pieces stay wet and sticky and never set, the syrup came off too early: boil it another minute next time.

Yes. The Kagome and Kurashiru recipes microwave the soaked pieces under cling film at 600 W for 3 to 5 minutes, then brown them in a pan with only 1 to 2 tablespoons of oil. Dry the pieces well after the microwave, or the steam makes the oil spit, and turn them often over low heat for about 4 minutes before switching to medium for the crust. Then make the syrup and coat as in steps 5 and 6. The result is a little less crisp than the shallow-fried version, but close, and it uses a fraction of the oil.

Daigaku imo is best the same day, while the shell is thin and crisp. Kept in an airtight container in the fridge it stays good until the next day, which is the guide Kurashiru gives; the shell softens and the syrup turns firm and a little sticky. Warm the pieces for about 20 seconds in the microwave to loosen the syrup, or eat them cold, as many people do from a bento box. Put in a slice of tamagoyaki and a small portion of rice and you have a lunch box. Do not cover them while still warm, or the steam softens the shell.

Japanese varieties with pale yellow flesh and a dry, floury texture are the usual choice: Beniazuma and Naruto Kintoki hold their shape and spit less in the pan. Sweeter, stickier Beniharuka also works if you start it in cold or low-heat oil. Orange-fleshed American sweet potatoes (Garnet, Jewel) are wetter and softer, so they break more easily; cut them thicker, about 4 cm, turn them gently and expect a softer middle. They are better kept for sweet potato casserole if you want a firm, crisp-edged result.