Shiratamako Flour is a version of sweet rice flour, milled by wet-meal-method. It is more expensive than mochiko.
To make it, grains of sticky rice are first soaked in water, then the soft rice is mashed, then freeze dried.
The flour is lumpy, even though the size of the powder grains is actually very fine. Some people like to whiz it in a blender before using to make it a fine powder.
When cooked, it creates a very fine texture. It also makes doughs come out both very stretchy, and sticky.
Substitute: per half cup of mochiko, add 1 tbsp of cornstarch or potato starch.
Shiratamako means "white jade powder."