What does a baby corn snake eat?

Corn snakes eat generally mice, or possibly rats if they are large enough.

I suggest the frozen variety. My reasons being, feeding live can increase the aggressiveness in the snake, it can also be potentially harmful to the snake if the mouse decides to fight back.

Frozen mice are also a lot easier to store, and to feed.

Make sure you are feeding outside of the actual cage, because feeding inside can also increase aggressiveness. Also if you feed frozen, make sure the mouse is completely defrosted, and warm (not hot) to the touch.