Katakuriko is a fine vegetable starch used as a thickener in Japanese cooking. The starch was traditionally derived from the katakuri plant (trout lily, see image lower on this page) but now is actually a term describing the more common potato starch.
You use katakuriko as a coating for fried foods or combined with water to make a paste used for thickening a stir-fry as well as other sauces.
You can purchase imported Japanese Katakuriko (potato starch) in most Asian markets or online at Amazon.com: Katakuriko Potato Starch
Combine equal parts of the starch with cold water. Whisk the mixture to combine and create a smooth lump-free paste. Use the paste immediately. You can make a thinner paste using more water to adjust the thickness of the sauce.