Get Domiyouji from the shop, this is mochi rice that has been cracked into pieces. Each grain of rice has been split in 4-6 pieces.
Soak this in 300cc of water overnight.
Then steam it for 30 minutes.
Mix it together with 210g of sugar (johakuto, moist sugar. Normal sugar works too). Add the sugar in 3 portions and stir between each portion.
Cover it up in a bowl with a damp cloth, and leave it for 10 minutes.
A mochi sweet like this should have about 25g of mochi and 25g of koshi-an inside.
Since this sweet is very sticky, it is normal to wrap them in a leaf of some kind. Cherry, tsubaki and oak are popular. The leaves is normally picked and soaked in a salt solution. Some of these leves you can eat, and others you should not. Cherry leaves are ok to eat. Profesional sweetmakers will keep these leaves for up to 1 year.