I must confess: I left a pound of butter on the seat of my car. It got unexpectedly warm that day and I forgot about the butter until the end of the day. The entire pound of butter had melted into the seat of my car, leaving the little paper cubes perfectly intact, yet empty, and the aroma of buttered popcorn wafted from the car. I mopped up what I could. Papers (and butts) in contact with that seat come away a little oily, and now it’s starting to smell bad! What can I do to clean this seat??
oh my. you are in a sticky situation. I have a feeling its soaked into the padding too. I see a new seat in your future
I bet its buttery soft now tho 
You need to remove the seat so the smell of rancid butter doesn’t inundate the rest of the interior.
Then you could take the seat to a local establishment that does vehicle interiors and see if they can save the seat.
Tester
I think Tester’s made a good suggestion. Expect that they may suggest replacing the foam and the pad. The leather itself can probably be saved and reinstalled once it’s removed.
For the record, when my kids were little I had an entire large box of crayons melt into my fabric seat in my minivan. I came back from a day in the park and all there was left was a Crayola box with lots of hollow tubes that used to hold crayon wax (64, if I remember correctly). Fortunately, it wasn’t a food stuff, but it did necessitate a new cover.
Aw, Jeez.