Caster sugar is finer than granulated sugar, but not so fine as icing sugar. Over here, we use caster in most cake recipes because it makes for a lighter texture. According to Yahoo!Answers *g*, you call it "superfine" or "Baker's" sugar.

Also, I tend to find that the number of eggs used in that recipe makes it very sticky and I seem to remember the first time I made it, it turned out more like a batter than a paste! I'd recommend using one less egg than she says. You can always add a bit if it's too dry :)