So after my first normal (without any secrets, only trying to talk to the black cat in the ice map) playthrough, I did another one in which I found the "hole to nothingness" and got the key when I got to the door at the end, the game took me to the menu screen, but it was different.
The text at the start was purple and was displayed on a black background, after pressing space, the camera moved to reveal the area where you normally end the game, but it was all black and purple and there were multiple variations of the face you meet as the "final boss" scattered around that zone.
I was excited and surprised to have found this and in my excitement, I chose to continue and spawned back at the place with the portals at the end of the game, but it was white and normal, I tried to get back to the menu to see if it was still there, but the menu screen was back to normal.
I then went on google to look for videos about the secrets of the game to see if I had missed anything by clicking "continue" instead of "play", and while I did discover a ton of secrets that I hadn't found myself, I found nothing about the black menu screen.
I then stayed up until 4:34 am, which is the time I'm writing this post, trying o recreate what I had done in order to get the menu screen, but I was unsuccessful, and, after all the time I spent on this without getting a result I decided to write this post.
I looked for a subreddit to write this on, but all I found were posts promoting the game, if anyone who sees this post knows of a better forum to post this on, please, do let me know, because I really do want to get to see that menu screen once more.