Place butter into the bowl of an electric mixer, beat until smooth, slowly add the icing sugar, mixing in well.
Add the flours and baking powder gradually, until all incorporated. Remove to a lightly floured surface and bring together with your hands. Roll out to ½ cm thickness and using a bread shaped cutter, cut shapes and place onto a lined oven tray. Refrigerate for 20 minutes before baking.
Pre-heat oven to 170°C (150°C fan-forced). Place trays of shortbread into the oven and bake for 20 minutes, or until just golden underneath. Allow to cool for 5 minutes on trays before removing to a cooling rack to cool completely.
For icing, place icing sugar in a large mixing bowl, make a well in the centre and add the butter. Pour in the boiling water, allowing it to melt the butter slightly. Mix well until smooth. Spread over cooled shortbread and sprinkle with 100’s & 1000’s. Allow icing to set, then serve.