Make it with water. Buttermilk won't really work for this. It's not quite the same. Buttermilk is essentially milk that's gone sour under controlled conditions. It's good for batters and dough, but not sauces.
FP
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Come away, O human child!
To the waters and the wild
With a faery, hand in hand,
For the world's more full of weeping than you can understand.