Yokan is a Japanese sweet made from agar, sugar, arrowroot powder, water, and adzuki bean paste. It ends up something like Turkish Delight. Sometimes additional flavourings such as fruit or chestnuts are added. Westerners tend to prefer these, finding that for them the plain ones have no taste.
You can buy it in Japan ready-made, sold in pieces packaged in plastic bags.
It's not always just sold solid, though: it sometimes can be bought in a liquidy form in a snack-sized can that comes with its own spoon for eating straight out of the can.
A Chinese version, called Pea Jelly, is made with split peas.
Cha Yokan (aka Matcha Yokan)
Yokan flavoured with matcha powdered green tea.
Yokan with pieces of steamed chestnuts (marrons) in it.
Yoru no Ume
Yokan made from paste from Dainagon Beans (a premium variety of Adzuki beans.)
Made from sweet potatoes instead of bean paste.
To make at home, you soak the agar-agar in water for half an hour, tear it into small pieces, then simmer until it has dissolved, then strain. You then add sugar and bean paste to the water, simmer for about 5 minutes, remove from heat, let cool and pour into a tray or mould. You then set it aside to let it firm up. It is cut into squares to serve.
Store indefinitely in refrigerator.
Yokan started to evolve as a dessert between 1600 and the mid 1800s (the Edo period), as sugar slowly became available to more cooks.