If your plasma workspace layout is locked, unlock it first: Right-click on desktop -> Unlock widgets.
Now open your application menu and right-click on the application you want to add a launcher for, then select `Add to Panel`: ![enter image description here](
You will now have a launcher which will launch a new instance each time you click on it and will stay there regardless of running instances.
That being said, personally I do use the Launcher icons that transform into an instance button in the task manager, but I configured the Task Manager to launch a new instance on middle-click:
![enter image description here](