I generally don't use a recipe I use a vague kind of idea from a cake I baked years ago and that is basically use good quality (salted is nice with carrot cake takes a teeny bit of the edge of sweetness of the icing sugar)and I use 4oz Icing sugar and 2oz butter. (or same ration in larger amounts dependant on how many i'm doing.) I don't melt or soften the butter using any heat just mash it up with a spoon and fork to get it creamy and gradually add icing sugar. Sometimes add a tiny spot of milk to help soften the butter while I'm mixing. Then add wee bit orange juice or even drop orange flavouring whatever you prefer.