These are cups that I designed and based on the Yin and Yang symbols.  A pair of these will cleanly mate together and form a circle.

I modeled these in Autodesk Fusion 360. I used the manufacturing (CAM) support to generate the CAM for machining them out of 6061 aluminum using a Haas VF 2SS mill.

More photos and info on my Fusion 360 gallery:

https://fusion360.autodesk.com/projects/yin-yang-espresso-cup

Cheers!