Hello forum.  I've been trying to accomplish this "simple" shape for some time... and have not been successful.  I was hoping you guys could point me in the direction of a resource or a technique that can help me. 

So the shape I am trying to achieve is essentially a "pillowed" cube. 

Ive designed a standard cube with soft, filleted edges.  (I dont have examples of my work, but it is exactly like the white cube in the attached pictured below).

I am not certain on the proper terminology...  But I want to achieve a slight "pillow" effect to the flat surfaces of the cube.  I cannot find an exact visual representation of what I am talking about, but the colored cubes below is close enough.  (Please ignore the fact that they are Rubix Cubes.  I am talking about the general shape)

Can anyone please help me figure out how to achieve this effect?

Thanks so much in advance.

-Hayden