How to connect a USB-C-based MacBook to a Thunderbolt display? Given the new 12" Retina MacBook doesn't have a Thunderbolt and only has a USB-C port, how do I connect it to an existing Thunderbolt display?
As @Kevin McKenzie already said, and just to expand on it.

At present there is no adapter for the USB-C to Thunderbolt.

Here is quote from http://9to5mac.com/2015/03/09/apples-12-macbook-gets-new-usb-c-power-vga-usb-and-hdmi-adapters/


  Thunderbolt Gen 1 and 2 don’t support USB 3.1, Type C or even 3.0 for
  that matter. An adapter wouldn’t have fixed this problem. However,
  Apple is releasing Thunderbolt 3 later in 2015. It’s not ready yet,
  because it’s developed with Intel and now Gen 3’s functionality is
  tied to Skylake chips – which will replace Broadwell shortly. I know.
  You’re thinking “But Broadwell just came out!” That’s true, but only
  because of delays. Now, you may wonder how a new MacBook without a
  Thunderbolt port or a Skylake chip will support a Thunderbolt Display.
  
  Well, in short it won’t. That is unless Apple releases a firmware
  update for their Thunderbolt cables. Which they won’t. So it probably
  won’t support it. Considering the last time they updated it, I think
  we’ll see an update at WWDC, with a USB C port on the display itself.