Marshmallow Powder can be used to describe two relatively different powders. Both are thickeners, though one is real and the other artificial.
It can be a powder from the actual root of the mallow plant. It is used rarely in modern cooking, though it was certainly used a great deal more in cooking as a thickener up till the advent of gelatin which replaced it as a less expensive thickening option. Now it is largely used in herbal remedies, especially as a binding agent in pills.
Or, Marshmallow Powder can be a commercial product (which has no real Marshmallow in it) used by ready-made food manufacturers. It is made from essentially the same ingredients as marshmallows are. It is used to add body and to thicken items such as meringues, etc.
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Powdered Marshmallow; Poudre de guimauve (French)