It's worth a quick reminder that Chromecast doesn't just mirror a browser tab onto your TV. You also can beam individual YouTube videos on over, same as you do from your phone or tablet. By now you've got the "Google Cast" extension installed, right? Once you've got it, just head on over to Youtube and fire up a video. Then look for the Chromecast icon in the lower right. Click it and choose where you want to send the video to. Done.
Get more tips and tricks in our Chromecast forum!