Buttermilk substitute in baking?

[Life is Good]

My favourite chocolate mudcake recipe calls for buttermilk. Not a lot - just enough to make me buy a huge carton (because that is the only way it comes here), use 1/5 of it and then let it sit in the fridge until it has little buttermilk babies of its own!

I am so over wasting and am just not game to use old buttermilk in what I regard as a relatively expensive cake to make (and potentially ruin!).

I am really not convinced about using milk with a bit of lemon or vinegar added (though I stand advised if you ladies have had success with it).

I am wondering if I could just use Greek yoghurt or sour cream? I imagine these would be a closer substitute? Again, what do you all think?

Do you think I could freeze the buttermilk into portions? Can you freeze buttermilk?

TIA

[Olive]

not sure about using other things, id just use the vinegar recipe, ive done that before without any issues

[Astrid]

You can freeze buttermilk. I have some frozen, but am yet to see if it works the same when defrosted.

[~Kim~]

I know Christy uses a combination of ricemilk and tofu blended together....probably not what you're after but there are alternatives.
And yes milk freezes well just takes ages to thaw.

[Rouge]

You can also make your own buttermilk with milk and vinegar or lemon juice. But like Astrid says I freeze my buttermilk in ziplock bags in 125ml lots.

[AnyDream]

Perfect excuse for buttermilk pancakes on the weekend!

I have used milk and vinegar in the past - works fine.