For the filling, add ingredients to a medium sized pot and place over medium high heat. Bring to a boil, then cook until thick and syrupy, about 5 minutes. Set aside and let cool.
Preheat oven to 375 degrees.
For the shortbread, cream together the butter and sugar until light and fluffy. Add vanilla and then mix in flour, cornstarch and salt.
Add 2/3 of the dough to an 8x8 pan and spread out evenly, pressing the dough into one even layer. Add the cranberry mixture and spread evenly, then crumble the remaining shortbread over the cranberry mixture. Lightly press down so none of the shortbread crumbles are too tall as they will brown more than the rest.
Place in the oven and bake for 30 minutes, until golden. Cool before slicing.