It can only be accessed via Level 14: Road Crash.
Go to Warp Room 3 and start up Road Crash. You’ll be driving a motorcycle in this level. At the start of the level drive slowly and stick to the left side of the road. On the left side there’s a yellow road sign with a black alien head on it. Knock over the sign with your motorcycle to trigger the secret exit. Hot Coco will now load. If you want to access the level again you must repeat this procedure.
A video showing the secret entrance to Hot Coco
Want more secrets? Check out the second secret level in the game called “Eggipus Rex“.