Chop up the sweet potatoes roughly, and soak in a bowl of water.
Put vegetable oil and granulated sugar in the frying pan, and lay the drained and pat-dried sweet potatoes (do not turn on the heat yet)!!
Turn on the heat to medium heat, and once the granulated sugar has dissolved, reduce the heat to low, cover with a the lid, then cook for about 5-6 minutes.
Remove the lid, flip the potatoes over occasionally, and cook them through. As shown in the photo, the surface of the sweet potatoes should be bubbly.
Tilt the frying pan and absorb any excess oil with a paper towel (it's just a small amount, so you should only need 1-2 sheets of paper towels).
Roast the sweet potatoes over medium heat, drizzle on soy sauce, then lightly shake the frying pan to coat them, and it's ready.
Garnish with black sesame seeds, if you like. (I didn't have black sesame seeds, so I used white sesame seeds instead...)
Note: The cooking time will differ depending on how thick you cut the sweet potatoes. They will cook faster if they're cut into bite-sized pieces, rather than large chunks.
Story Behind this Recipe
I had these candied sweet potatoes at my friend's house. My friend passed the recipe on to me, and I created this based on that recipe.
If you turn on the heat beforehand and try to lay out the sweet potatoes, the chances of burning the sweet potatoes are very high. Be sure to turn on the heat afterwards!! The amount of soy sauce depends on your taste, but I like it thick, resembling mitarashi sauce. The sweet potatoes will be crispier if you use granulated sugar instead of caster sugar.