Preheat oven to 180 degrees F 82.2 degrees C and line a square slice or baking tray with greaseproof paper (Tray dimensions 24cm x 5cm, 9 inch x 2 inches)
Add shortbread biscuits to the bowl of the food processor and blitz for 15 - 20 seconds to a fine crumb
Add the 100 grams butter to a heatproof jug and microwave for 1 min or until melted
Add blitzed biscuits to large mixing bowl, pour over melted better and use a spatula to stir to combine
Transfer to slice tray, spread, then use a flat based glass to flatten biscuit mix into one flat layer, place in the middle shelf of the oven for 8 - 10 mins till a light golden colour
Add sweetened condensed milk, brown sugar and butter to a heatproof jug, microwave for 3 mins, remove and stir to combine, microwave for a further 3 mins. Remove from microwave, place on a heatproof surface and stir for 2 minutes until consistently combined
Pour caramel over cooked biscuit base then top with macadamia nuts. Transfer to the middle rack in the oven and cook for 10 mins (till nuts are a light golden brown colour)
Use oven mits to remove slice from oven, place on cooling rack, then when cool, transfer to the fridge for 3 hours or overnight
Remove slice from fridge and use a sharp knife to cut into pieces. Store in a sealed air tight container till ready to serve