Leave the butter out to get to room temperature for about 30 minutes.
Cut the butter into cubes and add to a mixing bowl.
Add the sugar to the mixing bowl.
If using a mixer, use the creaming attachment and beat on low power until the mixture is light and fluffy. If beating by hand, cream the mixture with a metal spoon until it is light and fluffy.
Add the spice to the flour.
Gradually add the flour and spice to the mixture and continue to mix until it is a thick stiff batter.
Add the mixture to a 20cm baking tin, and with the back of a warm table spoon (heat it under hot water then quickly dry it off) press down the shortbread batter until it fills the tin and is even.
Using a fork, prick the surface of the batter all over.
Chill in the fridge for about 30 minutes. While it is chilling, pre-heat the oven to 160 deg C.
Cook the shortbread for about 25-30 minutes, until the surface looks dry and golden brown.
While the shortbread is still warm, deeply score (or cut through) the biscuit into squares.
Leave to cool for 10 minutes before removing from the tin, and breaking into the squares and placing on a rack to fully cool.
Store in an air tight container.