Place butter into the bowl of an electric mixer and beat until smooth. Slowly add the icing sugar, mixing in well.
Add the flours and baking powder gradually, until all incorporated. Fold in lemon zest. Remove to a lightly floured surface and bring together with your hands. Roll out to ½ cm thickness and use egg shaped cutter to press out shapes and place onto lined oven trays. Refrigerate for at least 20 minutes.
In the meantime, pre-heat oven to 170°C (150°C fan forced). Bake trays of shortbread for 20 minutes or until just golden underneath. Allow to cool for 5 minutes on trays before transferring to a wire rack to cool completely.
To decorate: dust each shortbread with a generous layer of icing sugar. With a small ice-cream scoop, place one dollop of lemon curd to resemble an egg yolk.