Action Comics#241(1958)

Whenever he needs to get away from the day-to-day acts of heroism, Superman retreats to his hidden Arctic Fortress of Solitude. Access to the Fortress is gained by way of a giant metal key – a key so heavy, that only Superman could ever hope to lift it. Inside his fortress are vaults and trophy rooms devoted to his various exploits. He has a room in honor of Jimmy Olsen, one in honor of Lois Lane, one in honor of his close friend, Batman, and even a room dedicated to star reporter, Clark Kent. One day, Superman visits a scientist named Professor Welkins, who asks the Man of Steel to test out the durability of a new metal compound he has been developing. Superman takes the compound back to his fortress and tests its breaking point. While there, he finds an ominous message written on the wall. Someone has managed to break into the Fortress of Solitude. The message taunts Superman, daring him to discover the intruder's true identity. In order to secure the Fortress from future break-ins, Superman welds the keyhole shut with a blast of heat vision. Superman flies off to put out a fire in Metropolis, but when he returns he finds a second mysterious message. Someone has found yet another way to invade his sanctum. Unable to piece the puzzle together any further, Superman retires to one of his fortress's laboratories where (enclosed with lead) he begins experimenting on a piece of Green Kryptonite. The intruder is still present however and soon makes his appearances known. It is Batman! As Batman leaps out and admits to leaving the vague messages, a cave-in erupts sealing the Man of Steel and the Dark Knight Detective inside of an enclosed cavern. Trapped inside with them is a deadly piece of Kryptonite. Superman is too weak to burst through the cave walls to generate an escape. As Batman begins to panic, Superman stands up, and reveals that the cave-in was actually a carefully staged ruse designed to flush him out. Batman tells Superman that in honor of the anniversary of Superman's arrival on Earth, he wanted to provide him with a fun, challenging game. He reveals that he managed to break into the Fortress by creating a secret stowaway panel inside the pointed tip of the giant key. The two go to the Batcave in Gotham City, where Batman has personally cooked a giant Anniversary cake for Superman. The cake is decorated with extra-large candles molded in the shape of Superman and Batman.