Preheat oven to 180°C. Line a large baking sheet with baking paper. Make a buttercream mixture by combining butter and icing sugar in a bowl and beating with an electric mixer until pale. Remove 2 tablespoons of mixture from the bowl and reserve for the filling.
Combine ½ cup of macadamias and the flour in the bowl of a food processor and process until you have achieved a fine flour. Do not over-process or the mixture will stick together. Stir thoroughly into the buttercream mixture.
Place the mixture in a piping bag with a star shaped nozzle and pipe small 1cm round kisses onto the prepared tray. Bake for 7-9 minutes until still pale but just turning golden on the bottom. Allow to cool on the tray for 5 minutes before carefully removing to a wire rack to cool completely.
Meanwhile, stir the extra finely chopped macadamias into the reserved buttercream mixture. Place in a clean piping bag with a plain nozzle and pipe a small amount onto the base of half the biscuits, sandwiching together with another biscuit as you go along. Serve immediately or store in an airtight container in the fridge.