Shiratamako is a finely milled Japanese glutinous rice flour. It is called for in traditional wagashi confections such as shiratama dango and nerikiri.
It is not quite the same as mochiko, which is also a glutinous rice flour. Mochiko is dry-milled, using rice grains that are washed, dried, and ground directly. It forms a smooth, elastic dough when hydrated and cooked. Shiratamako is wet-milled, made by soaking rice grains, grinding them, and straining the resulting slurry. The resulting flour is coarser and lumpier when dry, but once hydrated, it makes a smooth, silky dough.
Mochiko-based dough is dense and elastic, suitable for chewy rice cakes. Shiratamako-based dough is lighter, more delicate, and smoother, and is a good choice for soft but slightly resilient mouthfeel is desired.