Before heating, mix all the ingredients except for the tsubu-an. Simmer over medium heat for 4 minutes.
Turn the heat off, add the tsubu-am and mix.
Pour the mixture into a container and freeze.
I recommend freezing in ice cube trays. If you use a plastic container, you'll have to let it defrost for at least 20 minutes before eating. This is what it'll look like.
Story Behind this Recipe
It's too sweet if made with store-bought condensed milk and adzuki, so I came up with my own recipe.
I used homemade anko, which doesn't contain much sugar. If you use store-bought quality, reduce the amount of sugar. It has to be sweet to a certain extent or it won't taste very good, so add more if necessary. If you use 2 tablespoons of skimmed milk, you'll have an even milkier finish.