Preheat the oven to 180C/170C Fan/Gas 4.
In a large bowl, beat the sugars and butter together until light and fluffy, either by hand using a wooden spoon or with an electric mixer. Stir in the egg and vanilla extract. Sift in the flour, salt, baking powder and ground ginger and mix thoroughly to form a dough.
Mix in the chopped nuts, stem ginger and dulce de leche until well combined.
Line a 30cm/12in circular baking tray (such as a pizza pan) with silicon paper and then press the cookie dough onto it in an even layer about 1cm/½in deep. Alternatively, chill the dough and then shape it into a freeform circle on a lined baking sheet. Decorate the top of the biscuit with the whole macadamia nuts.
Bake for 25-30 minutes, or until golden-brown all over, but still soft in the middle. The cooking time will vary by a few minutes, depending on whether the cookie is in a tray or freeform, so keep checking it towards the end of the cooking time.
Remove the biscuit from the oven and leave to cool for a few minutes before removing from the tray and silicon paper. Leave to cool fully on a wire rack. Be careful when handling it as it will break easily.
In a small bowl, mix the royal icing sugar with enough water to form a paste. Add food colouring of your choice (divide the icing into separate bowls if using more than one colour). Spoon the icing into a piping bag fitted with a small plain nozzle.
Decorate the cooled biscuit with icing. Allow the icing to set then serve.