To make the Miso Honey Mustard Dipping Sauce:

Cup of Yum
1. In a medium bowl, whisk together the white wine vinegar, mayonnaise, Dijon mustard, sunflower oil, honey, white miso, and black pepper until smooth.
2. Adjust seasonings to taste.
3. Set aside or cover and refrigerate for up to 2 days if not using immediately.

To make the Sweet Potato Tempura:

1. Cut the unpeeled sweet potatoes into 1/4 inch (6 millimeter) thick discs. Pat dry on all sides with a paper towel to make sure no moisture remains.
2. Pour the oil in a large saucepan or wok to about 2 inches (5 centimeters) deep and heat to 360˚F (185˚C).
3. In a large bowl, beat together the water and egg yolk until frothy.
4. Whisk in the flour and cornstarch using chopsticks until just combined, but not completely smooth, no more than a minute.
5. Dip a sweet potato slice into the batter to coat completely, allow any excess to drip off, then place immediately in the heated oil. Repeat with more sweet potatoes, being careful not to crowd the pan.
6. If the batter is too thick when coating the sweet potatoes, mix in a little more ice cold water. If too thin and falling off, mix in a little more flour.
7. Fry until golden on both sides, 3-5 minutes, then transfer to a towel-lined plate or rack. Remove any small leftover fried pieces of tempura batter as well.
8. Repeat with remaining sweet potato slices, letting the oil get back to 360˚F (185˚C) between batches.
9. Serve immediately with the Miso Honey Mustard Dipping Sauce or Tentsuyu (Tempura Dipping Sauce).