Made from dried chestnuts, this is a light brown flour without gluten. Not suitable for making bread (unless mixed with a proportion of wheat flour), but it's excellent for cakes, biscuits, pancakes,...
If you intend to buy: You can sometimes find it in supermarkets, but more usually in organic stores.
If you want to use it: Chestnut flour must be sieved before use to prevent lumps in your recipe.
By "mash powder" do you mean mash potato powder?
If so, yes you can, see how to on this chicken recipe : chicken breasts in a potato crust.
Can I use potentiometer mash powder to make cutlets?