Melt the butter in a medium pot, and while the butter is melting, mix together all the dry ingredients.
When the butter is just melted, add your dry ingredients mixture and stir until all it comes together.
Spread into a 25x17cm (or similar) baking tin and prick with a fork. Bake for 1 hour, until shortbread is a golden brown colour.
Once removed from oven, cut into slices immediately and sprinkle with a little more caster sugar but leave in baking tin for 20 minutes before transferring to a wire rack to cool (this prevents the shortbread from crumbling and falling apart.
Depending on your oven, this recipe may be ready a little quicker, so check after 45 minutes.
I made these last night and they were lovely - melting the butter makes mixing a lot easier and less messy. I also added a rounded teaspoon of cinnamon which gave a really nice flavour. Thanks ;o) - 15 Sep 2013