This is the recipe royal chef Darren McGrady would make for the royal family at Balmoral. I modified his recipe to make it into round cookies.

225g (8oz) plain flour
225g (8oz) unsalted butter, cut into 1/2" cubes
115g (4oz) icing sugar  (some people prefer granulated sugar for shortbread)
115g (4oz) corn starch
1 tsp. salt

2 tsp. vanilla extract

Preheat the oven to 175° C/350° F.

Add all of the ingredients, except the vanilla, in a large bowl and work together until it resembles breadcrumbs. This dough will be really crumbly at first, but do not lose faith.

Add the vanilla and combine with the dough.

Tip out onto a lightly floured surface and gently knead to form a solid dough.

Roll dough out to about 3/8" and cut into 2" rounds.

Place onto a parchment lined baking tray.

Bake in the oven for around 20 mins until they are a light sandy color.
Once baked, remove from the oven and dust the surface with generously with granulated sugar while still hot. Shake off excess sugar and place on cooling rack for at least 1 hour.