Warning: Spoilers Ahead!Edit
"Find the key that turns dreams into reality and the entire world will open up for you."
The Gold Leaf Key is an item you find in Dreaming Mary and is essential to getting certain endings.
The Gold Leaf Key is found on the large, fluffy bed in the starting dream room. It is hidden in the chocolate. It can also be removed from the door to the dream hallway and the door to the nightmare hallway after being used.
The key is referenced multiple times in Dreaming Mary, including during the opening text (often theorized to be Mari's deceased mother) and by Penn Guindel, who asks Mary where she left it. This key allows the player to originally open the door to the rest of the dream world. However, if they can remove the key afterwords. If they find a nightmare room through exploring, which has a locked door, they can use it again, allowing Mary to enter the nightmare hallway. Remove it once more, and if the player reached the ending where they play as Mari, they can open the locked door and leave, with Mari assumingly escaping from her abusive father.