Preheat oven to 140°C (fan forced) or160°C.
Beat butter, sugar and vanilla in a large bowl with an electric mixer on high until pale and fluffy.
Whisk flours together.
Add flours and milk in two batches to butter mixture and beat on low until just combined.
Your flavour options are now added. Mix on low for 30 seconds. (see note for flavour variations I used)
Divide mixture in half and knead each half on a floured board until mixture comes together.
Roll each half into a log, the thickness of the cookies you want. Wrap log in baking paper and then cling wrap.
Refrigerate 1hr or until firm. If you intend to freeze for later do so at this step. Slice into 1cm thick pieces and place on baking trays lined with baking paper.
Bake for 20 minutes or until pale and golden. Leave shortbread on the tray for 20 minutes and then move to wire rack to cool.
If you are cooking from frozen you will need to add time to the cooking. In my oven I had to cook for 30 minutes at 140°c.