This is a K'NEX rotational mould which can easily be adapted to make easter eggs - i haven't got around to buying a mould off ebay yet - but it should work.
The concept is simple: after placing a hollow 2-part mould with melted chocolate in the middle into the machine, it will rotate in all dimensions allowing the molten chocolate to flow around the inside. As the chocolate cools, it will set onto the edge of the mould and when you open it, there will be a solid chocolate easter egg which is hollow.
This process is used to create many different kinds of hollow plastic objects from childrens toys to machine components.
Step 1: Firstly,
Step 2: Next,
The wheels that you can see on the bottom are just counterweights to help it rotate more easily.
Step 3: Finally,
Connect the squares to the frame. It is very important that the left hand side axel (connected to the motor) is secured to the squares so they turn together but the right hand side axel - connected to the two red gears and the chain does not move with the squares. this is so the gear connected to the frame stays still which is what makes the inner square rotate. (sorry if that wasn't very clear but hopefully the pictures, video and trial and error should help you get it right )