Preheat the oven to 180°C.
Beat the sugars and butter together in a large bowl until light and fluffy.
Stir in the egg and vanilla extract.
Sift in the flour, salt, baking powder, and ground ginger and mix to form a dough.
Mix in the chopped nuts, stem ginger, and dulce de leche until well combined.
Press the dough into an even 1cm layer on a lined 30cm circular tray.
Decorate the top of the biscuit with the whole macadamia nuts.
Bake for 25 to 30 minutes until golden-brown but soft in the middle.
Allow to cool briefly on the tray before transferring to a wire rack.
Mix the royal icing sugar with water and optional food colouring to form a paste.
Pipe the icing onto the cooled biscuit and allow it to set before serving.