
Wash sweet potatoes thoroughly under running water.
Scrape off the dirty skin with a knife and remove the whiskers. Cut into rounds of about ¼ inch/7mm thick to ensure they cook evenly in the the hot oil.
Soak the sliced sweet potato in cold water for about 10-15 minutes.
After soaking, drain the sweet potato slices and pat them dry with paper towels to remove any excess moisture.
Add cold water and Japanese mayo in a large bowl and mix well with a whisk. By thoroughly mixing the mayo and cold water first, you can prevent over-mixing after adding the flour.
Add the all purpose flour in two parts. If you over-mix the flour, it will become sticky and create gluten. Mix the first half with chopsticks until it becomes powdery.
Add the remaining flour. Mix together lightly with chopsticks. It's okay if some powder remains. Be careful not to over-mix.
Heat oil in a frying pan to around 338°F/170°C. *3
Use a brush to lightly coat the drained sweet potatoes all over with flour.
Dip each sweet potato slice into the tempura batter, ensuring it is fully coated. Allow any excess batter to drip off before placing the slice into the hot oil.
Add the tempura batter coated sweet potato slices to the frying oil and fry for 3-4 minutes. *4
Insert a bamboo skewer into the tempura. If it easily goes in smoothly, then it is cooked all the way through.
Once the sweet potato tempura is cooked, remove it from the oil using a slotted spoon or tongs. Place the fried slices on a paper towel-lined plate to drain excess oil.
In a small saucepan, combine the dashi, soy sauce, and mirin.
Bring the mixture to boil over medium heat for about 1minute.
Turn the heat off and transfer the dipping sauce to a serving bowl or individual dipping bowls.