I use spreadable butter and normal butter depends on what I have in.
If I use normal butter then I use double the icing sugar, if I use spreadable I use double and then add 2oz extra to make it thicker. It doesnt really change the flavour for me so its ok. If you make it up and put it in your piping bag in the fridge that should help.
I whisk by hand or using my electric whisk, both come out ok.