Save me a trip to the supermarket please! Chocolate icing recipe that doesn't use icing sugar.

[Dumbledoresgirl]

I have run out of icing sugar. I have a 7 inch chocolate sponge cake I need to sandwich together and ice on top. I have a small amount of butter, plain chocolate, small amount of granulated sugar (dammit why have I got so low on basic ingredients?) and pretty much anything else I might need, but only about 1 ounce of icing sugar. Any ideas anyone?

in this weather it soud be fine at room temp - it's not tht it goes off but if it gets too hot it goes runny

Unless you live somewhere exotic...overseas rather than here (!) and your house isn't tropically hot, the cake will be fine.