The much-requested option to start a Chromecast stream from Spotify's desktop app is now available. It works from both Windows and Mac desktop apps. Previously, you were only able to control Chromecast streams that started from Spotify's Android or iOS apps using Spotify Connect. Now you no longer need this workaround. It's available in the desktop app version 1.1.38. All you need to do is head to the "Devices Available" icon to see available Chromecast devices.
Source: 9to5Google