Confirmed it’s a permissions issue.
If I run the editor directly (via UE4Editor.exe) with Admin privileges, I can Cook and Build correctly.
If I don’t elevate permissions, I still get the same problem (even if running the exe directly)
However, if I make UE4Editor.exe always run with Admin (via Compatibility settings), it won’t open via the Epic Games Launcher.
So I have a workaround, but there do seem to be some strange interactions with Windows 8 permissions/admin privileges.
I think this has only happened due to a recent Windows Update, as I was previously able to Cook/Build.
For anyone else with this issue (running Windows 8), try going into the UE4 install (ie: C:\Program Files\Epic Games\4.7\Engine\Binaries\Win64), right click on “UE4Editor.exe” and select Run as Administrator.
That should let you build/package normally.