Philidor 3. ... f5 4. exf5 Bxf5!?
This article is about the position in the diagram please do not comment "just" to complain about the countergambit it arises from:
Every source I could find on the counter gambit said that Bxf5 lost a pawn and left it there as "bad" so I had to explore it:
clearing up loose threads
The main idea is to capture on c2 with the bishop to even pawn numbers. Any unexplored lines are probably transpositions or semi-transpositions (positions that aren't quite the same but need the same ideas and moves). Although it is true that White can end with an advantage in some positions, those are few and there is some counterplay there. Of course, I made the theory so it might have a lot of holes in places or just be missing moves. I think White can maintain an advantage easily be protecting the pawn enough to force Black to recapture with the pawn, and then leave the pawn isolated and take care of the c-pawn.