just wondering how to get a really light, more importantly - smooth - buttercream? I've tried it a couple of times but it always feels a bit heavy (maybe too much sugar?) - maybe I'm just not such a fan of it...

I also wanted to know if I made a chocolate one, would I just add cocoa to a normal buttercream mix?

And lastly, some recipes call for milk, which I have added before - wouldn't it mean this needs to be served then & there to avoid refrigeration or is it ok out of the fridge for some time?