As others have mentioned, a torch is necessary. Also grind the tip of your Allen key, or socket flat on the tip to get a better bite. I initially tried other suggestions I read on here of using a cold chisel on the outer edge of the plug to try to rotate/break it free, but until I broke out the torch, it was just making a mess of the plug and deforming it. Once I found the chisel wasn’t budging it, I promptly ordered a new replacement plug and put off doing the change until I had it in hand. The torch IS the answer, but I wouldn’t recommend propane as it won’t get it hot enough. You minimally need mapp gas, or better yet oxy/acetylene. Don’t reuse use the old plug and washer, just get a new one. Also, I added anti-seize lube to the replacement so hopefully it will come off next time without as much fanfare.