One possibility is to tape a sheet over the whole window for about three weeks, so it disappears from her consciousness by blending into the wall, and/or so she gets the illusion of protection from things out the window. Another, if you want her to be able to look out the window and see something interesting, would be (while she is watching) to put a mirror in the window, facing in (in the daytime). Then the two of your can look into it (also in the daytime) from in the house. Keep it there for two or three weeks, and look into it every now and then. It might help her see that all kinds of stuff happens out of windows, but that does not mean anything scary.
As far as lifelong phobias, my son used to be afraid of the hair dryer and the vacuum cleaner, but he sure isn't now. So it is possible that after a few years she will like Halloween just as much as the next kid, even if she was frightened at age 1.
I think it would be good to make a game out of allowing her to look into the window at you or daddy and vice versa. She's young enough to replace the scary face to being mommy or daddy and may just forget about the scary one. Start with doing this during the daylight hours and move to night time. Make it fun for her. Also, try taking her outside when it's dark with the lights on so she can see that it's okay, and nothing will hurt her. I think she needs to be familiarized with the darkness, and learn that there's nothing scary outside.
Her last memory was of a scary face, show her that it can be daddy, mommy or even her. Good luck!
Ugh, sorry you're having to go through this.
Was the mask a specific scary character, or was it just the mere fact that it was a mask that frightened her? I know kids that age often have serious trauma after meeting clowns or other characters that are not intended to frighten - it's just the horror of a distorted face that's awful. God knows why there are so many children's activities and characters that involve distorted faces and masks.
My oldest son had the beginnings of a phobia of sheep at around that age (sheep??? I know, unusual, but he was jumped by this ENORMOUS bleating sheep at a fair) and the therapist said to "rest" sheep completely for 6 months. No mention of sheep, no attempts to reunited him with liking tiny little lambs, no baa baa black sheep, etc.
Do you have another door to your house that you can use - and completely block this one with a large sort of non descript thing like an armoir or refrigerator box? Completely obscure the door and leave the hallway brightly lit? I think at her age 3 weeks or so should do the trick and if you "rest" this fear by obscuring it and not making any attempts to show her the door again or reunite her with character faces, she will probably do well.
But yes, consult a therapist for ideas.
My son is 21 now and said he will never actually like sheep. ;D Your daughter will probably one of those kids with vivid imaginations who doesn't like costumed characters.