Creating an One Click button
This has been tested and worked on Ubuntu 18.04.2
Install a necessary package in order to easily create a desktop application. We will use the terminal because we need to use a special parameter to avoid unnecessary packages from being installed.
sudo apt update
sudo apt install --no-install-recommends gnome-panel
Call the app from the terminal, as follows
gnome-desktop-item-edit ~/.local/share/applications --create-new
In the opening window give a Name, Comment and an Icon of your choice.
The important here is the Command.
Copy-paste the following in the Command field:
dbus-send --system --print-reply --dest=org.freedesktop.login1 /org/freedesktop/login1 "org.freedesktop.login1.Manager.PowerOff" boolean:true
Add the application wherever you want on your Desktop. The time you click the button, it will shutdown the PC without further questions.