Charge MacBook 12" while connected with Google's USB-C to DisplayPort cable I have a MacBook 12" that I connect to my Dell 4K screen with a Google USB-C to DisplayPort cable (this one)

It works perfect. But now I am not able to charge the MacBook while connected because the cable takes the only USB-C port available.

So I need some kind of USB-C or a adapter that puts voltage on the cable back to the MacBook. Both I did not find yet.
Take a look at these two multiport adapters from Apple:


USB-C VGA Multiport Adapter (USB-C to USB-C/VGA/USB)
USB-C Digital AV Multiport Adapter (USB-C to USB-C/HDMI/USB)


There's no technical reason why third parties won't come out with adapters since all of this is standard. USB C charging, video formats, the connector type are all non-proprietary. Only time will tell if businesses invest in making these products, but Apple's two adapters work very well for the use case you describe.

Also keep an eye out for cables and monitors that send the required USB charging current over USB-C most higher quality displays should soon be able to charge your computer like Apple's previous generation displays now that the proprietary charge connection is gone from the MacBook (and others).