In a medium mixing bowl, cream together the butter, sugar, vanilla and lemon zest until smooth and creamy.
Sift the flour, salt and semolina into the bowl, then add the cranberries. Mix together to create a smooth dough. It may seem a little dry at first, but it will come together. (Tip: If you find that it is extra crumbly this could be because of the water content in the butter. Avoid overworking the dough as it will cause the shortbread to be tough)
Place a large piece of clingfilm on the work surface and scrape the dough out of the mixing bowl onto the clingfilm. Shape the dough into a log about 4cm (1½ inch) in diameter. You can use the clingfilm to help shape the dough rolling it up at the ends.
Place it in the fridge to chill for at least 1 hour.
Preheat the oven to 190°C / 170°C fan / 375°F / Gas 5. Line two baking trays with baking paper.
Unwrap the chilled log and using a sharp knife, slice each roll into approximately fifteen 6mm (¼ inch) thick slices. Place the slices on the prepared baking trays, allowing space around them.
Baking: Bake for 14-16 minutes until they are just lightly golden. Once baked remove from the oven and allow them to rest on the tray for 5 minutes before transferring them to a wire rack to cool completely.