Preheat oven to 150℃ (130℃ fan forced). Line the base and sides of a 20 x 30cm baking tin with baking paper.
Place butter, flour, sugar, cornflour, vanilla and 1/2 cup of the macadamia nuts into the bowl of a food processor. Process for 20-30 seconds or until a crumbly dough forms. TIP: To test if it's ready, pick up a handful and squeeze it. It should hold its shape.
Transfer the dough to the prepared pan and spread out as evenly as possible smoothing down with the back of metal spoon. Prick all over with a fork and bake in the oven for 1 hour or until lightly golden.
Remove from the oven, and while still warm (in the tin still), cut into 24 shortbread fingers with a sharp knife. Allow to cool completely in the pan.
Meanwhile, place chocolate in a microwave safe bowl. Melt chocolate in the microwave in 30 seconds bursts, stirring after each 30 sec time, until melted and smooth. Finely chop remaining macadamia nuts.
Line a large baking tray with baking paper and place a wire rack over the top. Transfer shortbread fingers to the wire rack. Working with one row at time, drizzle with chocolate then sprinkle with macadamia nuts. Repeat until all decorated. Allow chocolate to set before serving.