Rancid butter added to food.
Just would like to clarify:) one thing, intense butter flavor is not the same thing as rancid butter. Butter should not have the slightest hint of rancidity. If you are served a rancid butter at a restaurant you should send it back!
For a better understanding of what rancid and oxidized butter is, read Johnny Drain’s excellent series of articles:
Yes, I admit that I like to age butter in peat bogs with the help and guidance of Ben Reade:
But, again, do not accept rancid or off tasting butter at restaurants!
To me, there is a gastronomic use of rancid butter and that is to use it in cooking. Heat the butter and the rancid off flavors will evaporate. The resulting butter flavor is quite nice and will give you complex umami notes. However I also recommend that you add a knob of fresh butter in the sauce or casserole. This last fresh butter should not be heated too much, otherwise all of the wonderful buttery taste components will disappear. So, actually you could ask the restaurant to heat the rancid butter you were served to exactly 46 degrees Celsius for 60 seconds and then serve it. Dip your bread in it.