Preheat he oven to 150°C. Whizz all the ingredients apart from the stem ginger in a blender until it comes together. Turn the dough out onto a floured surface and briefly knead in the stem ginger.
Roll out to 1cm (½in) thickness; shape into an 18cm (7in) square. Put in the fridge for 1 hour to firm up. Cut into 12 fingers.
Bake the shortbread for 15 minutes. Remove from the oven and loosen the biscuits from the baking tray. Leave to cool for about 5 minutes, before transferring to a rack to cool completely.