• 25 gr わらび粉
• 170 gr water
• 80 gr sugar

Mix わらび and water, strain into pot, add sugar. Heat and stir for about 7', pour onto きな粉. Wrap around あん (20 gr balls), makes 10.

I found it works also well with うぐいす粉, and it's nice if you use あん made with black sugar (because in tea ceremony, you wouldn't add the black sugar syrup).

In German, わらび is Adlerfarn. It's very common, and it's poisonous (leaves, roots and all). So while there are videos of people making their own わらび-starch by digging out the rhizomes, crushing and washing them and then letting the starch settle; it's probably better to get store-bought わらび粉 (or a substitute with cornstarch).

You'd dig out the roots in autumn, in any case, but some Japanese dishes use the young shoots of the plant in March (again, poisonous). Because of that, it's associated with March and spring time (even though the roots are harvested in spring).