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Traditionally, they are made with just 金団-ing agar cubes around a ball of white あん, then brushing or drenching the whole thing in an agar bath. They don't look very elegant, however, and are a paint to transport.
So this I have from a book. It's a little bit involved since it uses heavily coloured あん that is supposed to shine subtly through a layer of white あん. But I think even without the colour work it is much more preferable to have the jelly sit in its crater rather than going everywhere:
Recipe
For the gelly, mix 1 gr of agar, 70 gr of sugar and 100 gr of water. Boil briefly and reduce slightly, until you can form drops off a spoon.
The Migros' brand is 22% agar, so here you'd use 8 gr of the mix (= 1 pack), 112 gr sugar, 160 gr water.
Make drops (which you can also use for any other summer sweets): Drop on baking sheet, that's it.
Colour slightly, place into shallow container and rest in fridge for 1 hour before cutting.