I dont think the Manx were into Cookies (sounds very American!)
try Furmity:
Take shelled barley, and boil it in water about an hour. Then put some sweet milk in it, then stir a little flour in cold milk, and put them in the pot, and stir them till they will be brought to boil, and boiled well.
(see http://www.ee.surrey.ac.uk/Contrib/manx/fulltext/mr1911/ch12.htm)

Or of course Bonnags - a circular flat loaf. ( - Of cakes we have the Manx bonnacks, and the arran oarn, arran corkey, and arran flooyr (ie with Barley, oaten, or flour cake.)(see http://www.ee.surrey.ac.uk/Contrib/manx/iomnhas/lm3p129.htm#food)
Most bonnag I see is more like an English/Irish scone recipe but think it was actually nearer plain unleavened bread (maybe a bit thicker than a chappatti ?) - dried fruit, butter and some spice seems to find its way in - I have a recipe at home but suspect any one with Susan Wooleys book will find a modernised recipe