Culinary Notes
The Cornstarch Factor: Because of that added starch, Icing Sugar is not just a sweetener; it is a slight thickener. This is why it helps "set" a buttercream or a glaze. However, it also means you should never use it to sweeten hot drinks (coffee/tea), as the starch will leave a nasty, floating scum on the surface.
Sifting: Icing sugar is prone to forming hard little lumps over time. You must always sift it before adding it to a delicate frosting or dusting a cake, or you will end up with visible white rocks in your chocolate finish.
The "Melting" Garnish: If you dust icing sugar onto a warm cake or moist tart, it will absorb the moisture and disappear within minutes. If you want a permanent white dust for display, you need "Snow Sugar" (which has added fats to repel water) or you must wait until the cake is completely cold.
Shortbread Texture: Using icing sugar instead of caster sugar in shortbread dough yields a "melting" texture rather than a crisp/crunchy one, as the fine powder integrates more completely with the butter.
