By using Egg Cuber, we can create cube-shaped eggs.
From the instruction,
Put warm, peeled, hard boiled egg in EGG CUBER with base plate in bottom. Place forming plate on top of egg.
Screw top down until egg becomes square. (Best results if EGG CUBER has been chilled).
Unscrew cap and push base plate up from the bottom with finger. Remove square egg.