First, add your peeled and cubed sweet potato into a pot of water. Place on high heat and boil for 8 minutes or until soft. Drain and set aside.
Meanwhile, make your dough by combining the flour, oil, salt, and boiling water in a bowl. Stir with chopsticks while you pour the boiling water into the bowl. You should end up with shaggy dough. Pour it onto a clean surface and then knead for 3 minutes. Set aside covered in cling film.
Grab your sliced leek and add it to a frying pan with a glug of vegetable oil. Add in your cooked sweet potato, 1 tablespoon soy sauce, ½ tablespoon sesame oil, and ½ tablespoon sugar. Combine thoroughly until a chunky paste has formed. Turn off the heat and set it aside.
Knead your dough again for 1 minute and then cut into 8 equal chunks.
Roll out each piece into a thin circular wrapper, around 3 inches in diameter.
Place a spoonful of the filling into the centre of a wrapper and crimp up in a circular motion. Make sure the dumpling is sealed so none of the filling can come out. Pat the dumpling down slightly to resemble a fat, disc-like parcel. Repeat this step until you have no dumpling wrappers left.
Pour some vegetable oil onto one plate, and some sesame seeds onto another. Place the smooth side of the oyaki down into vegetable oil, and then in sesame seeds. This is optional, but it makes the dumplings look cute!
Grab a frying pan, place it on medium heat, and drizzle with a bit of oil. Gently add the dumplings sesame seed side down into the pan. Fry for 3 minutes or until slightly golden, and then flip onto the other side. Pour in 30ml of boiling water, 1 tablespoon soy sauce, and 1 teaspoon sugar, then place the lid onto the pan and leave to steam for 8 minutes. Keep an eye on the pan, to make sure that the dumplings don't burn. If the liquid has evaporated before the time is up, simply add a bit more water.
Once cooked, remove from the pan, serve up and enjoy!