Preheat the oven: preheat the oven to 180°C / 350-375°F / Gas Mark 4-5.
Baking dish: line a square baking dish (24 cm x 24 cm or 9.4 inches) with 2 overlapping layers of baking paper with overhang on each side.
Sugar: add the brown sugar to a large mixing bowl.
Butter: add the butter to a heatproof container and place it into the microwave. Heat it on high for 30 second increments until melted (around 1 ½ minutes). Once melted, gently stir the butter.
Butter and sugar: next pour the melted butter into the bowl with the brown sugar and use a spatula or wooden spoon to stir until combined. Once combined let cool for 1-2 minutes.
Eggs, vanilla and flour: after letting the batter cool for a couple of minutes add the eggs and vanilla. Stir to combine consistently and then add the flour and stir again till consistently combined.
Mix ins: next pour the white chocolate chips and macadamia nuts into the bowl and gently fold in.
Transfer: use a spatula to transfer the blondie batter into the lined baking dish. Then gently level so that it is pushed into all 4 corners of the baking dish.
Bake: place the baking dish into the oven and bake the blondies for 35 minutes.
Cool: when the cook time is up use oven mitts to remove the baking dish and set it on heatproof surface. Let the blondies cool in the dish for at least 20 minutes. After this time is up, grab the baking paper and pull the blondies up and out of the dish. Place them onto a wire rack and let them cool for a further 20 minutes before slicing.