Preheat oven to 170 C/ 325 F and grease and line an 8-inch square baking tin.
Melt the butter in a small saucepan over a medium-low heat.
Sift together the remaining ingredients in a large bowl and pour in the melted butter.
Stir well to combine. Knead for a few seconds until it forms a soft dough and press into the base of the prepared tin. Prick the shortbread all over the top with the tines of a fork, sprinkle with caster sugar and bake in the centre of the oven for 25-35 minutes, until golden and crisp. While the shortbread is still warm, score out squares or fingers and leave to cool completely before removing from the tin.
Alternatively, roll out the dough between two sheets of greaseproof paper to a thickness of 1 cm. Chill in the refrigerator for half an hour and then cut the dough into your desired shapes. Sprinkle with sugar, prick with a fork and bake in the centre of the oven for 25 minutes, until crisp.
Store in an airtight tin.
Mum of three, daydream adventurer, OU Life Sciences graduate, occasional crafter, ex-pat Canadian & quite possibly Britain's most northerly food blogger.