Preheat oven to 150°C, 130°C fan forced. Line the base and sides of a 20 x 30 cm baking tin with baking paper.
Place the butter, flour, sugar, cornflour, vanilla, and ½ cup macadamia nuts into a food processor. Process for 20 to 30 seconds until a crumbly dough forms. To test, squeeze a small handful of dough. It should hold its shape.
Transfer the dough to the prepared tin and press it out evenly. Smooth the surface with the back of a spoon. Prick all over with a fork. Bake for about 1 hour or until lightly golden.
Remove from the oven and, while still warm in the tin, cut into 24 shortbread fingers using a sharp knife. Leave in the tin to cool completely.
Place the chocolate in a microwave safe bowl. Melt in 30 second bursts, stirring between each burst, until smooth. Finely chop the remaining macadamia nuts.
Line a tray with baking paper and place a wire rack over the top. Arrange the shortbread fingers on the rack. Drizzle with melted chocolate and sprinkle with macadamia nuts. Allow the chocolate to set before serving.