Combine the water, agar-agar, and brown sugar in a saucepan.
450 g water, 2 g agar-agar powder, 15 g brown sugar
Heat, stirring, until fully dissolved.
Bring to a boil and boil for at least 1 minute (this step is essential for the jelly to set).
Add the salt.
0.5 g salt
Press the koshian through a sieve, then stir it into the mixture until smooth, as you would with miso.
250 g koshian (smooth red bean paste)
Cook, stirring, for about 15 minutes, until slightly thickened.
Cooling, setting, and serving
Let cool for about 20 minutes, stirring to release the residual heat (this prevents the mixture from separating into two layers).
Pour into a rectangular dish.
Refrigerate until completely set.
Cut into portions and serve well chilled.
Notes
Important: Stirring the mixture as it cools is essential; otherwise, the bean paste and agar jelly will separate into two layers.Sugar: About 15 g gives a sweetness reminiscent of traditional confectionery, while 30 g produces a firmer, more reliable set that is easier for beginners.Variations: Add chestnuts, or make a version with salted cherry blossoms by pouring a second cooled jelly (200 g water, 2 g agar-agar, 20 g sugar) studded with desalted cherry blossoms over the fully set jelly.