Preheat oven to 160°C/325°F. Line a baking tray with non stick baking paper.
In a stand mixer (or with a handheld mixer or simply with a wooden spoon) beat the softened butter until smooth and fluffy.
Add the sugar and beat well until light and smooth.
Beat in the vanilla then stir in the flour, 1 cup at a time until fully mixed in. The dough with come together in a mass. Don't over beat. Turn out onto a lightly floured surface and knead briefly.
To roll the shortbread out, dust the work surface with flour and roll the dough into a rectangle about 30cm x 23cm (or about 12in x 9in). Square up the edges with your fingers. Cut into 18 pieces (cut the dough into 3 lengthwise and cut each strip into 6 pieces. These are big shortbread!
Prick each biscuit with a fork 3 times. Arrange on prepared baking trays.
Bake in the preheated oven for 25 minutes.
Allow to cool on the baking trays for 5 minutes before removing to rack to cool.