1. Line a 20cm x 20cm baking tin with parchment paper.
2. Cream the butter and sugar together before sifting over the plain flour and rice flour.
3. Add the cinnamon powder and lemon zest (if using).
4. Mix well until there are no traces of flour left.
5. Transfer to the lined baking tin and press down until flat and smooth on top.
6. Prick the top with a fork and pop in the fridge to chill down while you preheat the oven to 170°C/350°F/gas 4.
7. Remove the shortbread from the fridge and sprinkle with a little sugar. Bake for 20 minutes until light and sandy in colour.
8. Remove from the oven and score into long pieces. Allow the shortbread to cool in the tin for 5 mins before transferring to a wire rack to cool completely.