Buttercream icing disaster (wails loudly, first world problem)

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I'm not a butter fan but I discovered buttercream was actually really nice when I tried it in my Kenwood stand mixer (about 8 minutes mixing) and unsalted butter.

But I made two batches today (DD School Fayre) and it didn't work.

Soft butter, out of the fridge for a few hours.
First lot was too soft and buttery. I added a couple of spoons more icing sugar and it went thick and oily.

Tried again with new butter and icing sugar. Put a drop of milk in and whisked. It was a slminy mass not light and mousse-like.

My kitchen isn't hot. The bowl and beater (silicone K beater) had been in the dishwasher the night before so not hot.

I wasted 8oz of butter and 16 oz icing sugar.

Baaah .
What went wrong?

if it were me I'd stick it in the fridge to allow the butter in the mix to set up a small bit and give it a stir. I bet it would be okay then. I don't think you'll ever get a mousse like result though, buttercream is always going to be heavy ish and buttery!

More icing sugar until its almost too stiff to beat. I add a tbsp of trex when I'm making it, it seems to stabilise the mixture and makes it smoother. It doesn't affect the taste.

are you doing it at a ratio of 2 parts icing sugar to 1 part butter?

I'd say you overbeat the first batch; stick it in the fridge to cool down and try again later.

The second time did you add the milk at the start? I'd wait till the end, personally.

Thank you all.
It's just so blimming annoying because if I'm doing cakes for the DC my icing is nice and fluffy (has a dry almost tiny bubble texture, very easy to pipe) but today when I really needed it to work and I got up early to ice the cakes.............well, it didn't.

I do 2:1 icing sugar:soft unsalted butter and some vanilla extract.

Now I've got naked fairy cakes - I used all the icing sugar- so I feel a trifle evolving.

I use a 1kg box of icing sugar to 250g block of butter, then enough milk to make it smooth. I've used it for years (I sell cakes) and its always light and fluffy and creamy. If you mix it long enough (8-10 mins should do) it'll go almost white

I use salted butter and a little vanilla essence. I beat by hand, cream the butter and slowly add the sugar at first in small amounts. Increase the amounts of sugar as the mixture can cope with it if you see what I mean. When the mix gets too stiff to beat add water not milk, it keeps it lighter.

I use just over twice as much icing sugar as fat, and about a teaspoonful of milk - BUT I have discovered that (really unexpectedly) buttercream icing is actually nicer and has a better texture if you use about 2/3rds butter to 1/3rd soft margarine e.g. flora or soft stork. I normally can't abide marg but it really does make a difference in this case.