Slice your sweet potato into rounds, each about 2 cm thick. Peel off the skin, making sure you get all of it off since you want to give your yokan a smooth consistency. Let your sweet potato slices soak in a bath of water. Make sure to change the water several times.
Line a pound cake mold with baking paper.
Steam your sweet potato until soft (it takes me about 10 minutes in a pressure cooker). Add soft brown sugar, butter, and and natural salt, then combined everything with masher.
Continue mixing with your masher until the potatoes become starchy and sticky. Adjust to taste with sugar and salt.
Fill the pound cake mold with your mashed sweet potato. Let cool for 2 hours in the refrigerator.
It should look like this. Flip it over so that the smoother side faces up. Slice off any unevenness from each of the sides with a kitchen knife. Portion into your preferred sizes.
Story Behind this Recipe
I had a sudden urge to eat sweet potato yokan, so I decided to make some at home. The really smooth kind that you get in stores is nice too, but I think the texture of these is just right.
It's important to let the sweet potatoes soak in clean water to let all of the lye out. It's also important to peel all of the skin off, so that you don't get bits of it ruining the texture of your yokan. When you're adjusting the amount of sugar, it might be a good idea to make it a touch on the sweet side, since it'll taste a little less sugary when chilled.