black butter 
noun [mass noun]
A sauce made by heating butter until it is brown and, typically, adding vinegar.Skate with capers and - a rare, unconventional touch - olive oil rather than black butter is available....
- For skate in black butter, you begin by poaching the wings in salted, lightly vinegared water with herbs and aromatics for ten minutes until it becomes opaque, then drain it.
- The skate was served with traditional black butter; meaty and moist, I wondered at this point whether ‘flat-fish’ was a misnomer, or perhaps this fella was just on the fat side.