Can I use an edible food painting pen to decorate shortbread biscuits or can it only be used on icing?
From the Wilton website:
Draw edible messages and designs on molded candy or dipped treats! As easy to use as a normal marker, you can use these edible ink markers to add fun and dazzling color to fondant, icing, cookies and more. You can even brighten everyday foods like toaster pastries, cheese, fruit slices and bread.
So yes, you can draw on shortbread.