Arrowroot Powder comes from the dehydrated and ground arrowroot tuber and not from cassava root. It is great for adding lightness to a recipe and can also be used to thicken sauces. Arrowroot powder is so useful in recipes that I actually use the Subscribe&Save program from Amazon to have it automatically shipped to me! It can replace corn starch in a recipe 1:1 and you can also mix arrowroot powder with very finely ground granulated sugar to make an acceptable substitute for icing/confectioner’s sugar. If replacing wheat flour with arrowroot flour to add lightness to a recipe, replacing up to ¼ of your flour with arrowroot is typical. I love using arrowroot in conjunction with coconut flour for cake recipes. Just be aware that it doesn’t add much hold to baking that doesn’t have many binding ingredients, though.
Arrowroot is essentially a pure carbohydrate, and it doesn’t provide any significant micronutrition. Most recipes include very little (just a couple tablespoons at most), so most people don’t worry too much about the additional carbohydrates.
You can find this in the baking section of your local grocery store, but in some smaller areas it may only be found in health food stores. Or you can do what I do, and order it from Amazon.