
Put the yokan and water in a small saucepan and soak for about half an hour. *2
After it has soaked, place the pan over medium-high heat to bring it to boil. Whisk the mixture well.
Turn the heat down to low, continue whisking and let it simmer for at least a minute until the kanten dissolves completely.
Add sugar to the mixture and stir until it's fully incorporated.
Stir Koshian in until it's well combined and thickened, scoop it up with a spatula and if it is about 2mm thick and flows, turn the heat off.
Pour the mixture into a rectangular container. In Japan we use one that is called "nagashikan" in Japanese. *3
Chill in the fridge for about 2 hours.
Slice to serve.