When I dust my Victoria Sponge with icing sugar (when cold), it disappears within a few hours. Is there anything else I can use or do to make the decoration last?
Icing sugar is very fine, so it will tend to dissolve fairly easily. If the item being dusted is very dry then the sugar should remain for several hours, unless the weather is very humid. Unfortunately Victoria Sponge cakes are slightly moist and the surface of the cake can be slightly damp (particularly if the cake has been stored in a tin or a container) which tends to accelerate the sugar dissolving.
If you can then dust the cake with icing sugar just before serving. You could also dust the surface of the cake with caster sugar which has larger crystals so will dissolve far more slowly. You can also buy coloured decorative sugars from some baking websites which should last a day or two and could be a colourful alternative.