Keep putting penetrant on as it cools and it may pull some deeper into the threads. Work it back and forth gently- don’t go full retard on it. I had many come out on the first try, some take three tries. If its that stuck, then most other methods would not even come close to getting it loose. Last ditch effort you can try a left handed drill bit that is close to the minor diameter of the threads, and then retap the threads to clean up, but it’s hard to drill that without messing up the threads in the process.