Pre-heat your oven to 180°C (350°F, Fan 160°C, Gas Mark 4). Then, grease the inside of the baking tray with butter, before lining the bottom with baking paper. The butter will help the paper to stick, while leaving the sides exposed will enable those crunchy edges that some folks gravitate to
Fill the small saucepan around a quarter way with water, and put it on a low heat. Then, place one of the medium size bowls on top, so it rests on the rim of the pan, and doesn't touch the water
Add 200g of Butter and 200g of Dark Chocolate, broken into small chunks, and wait for it to melt
Meanwhile, put 90g of All Purpose Flour, and 50g of Cocoa Powder into the second bowl, mix together, and set aside
Once the chocolate and butter have melted, turn off the heat, and give it a quick whisk to form a delicious looking sauce. But, leave it on top of the pan so it doesn't cool down too quickly and solidify
Next, place 225g sugar, 150g Eggs, and 4g Vanilla Extract in the mixing bowl, and blitz it with the electric hand mixer until light and creamy
Now, check the temperature of the chocolate with a finger, it should be warm to the touch, but not hot
Once satisfied, slowly add the chocolate mix into the mixing bowl with the sugar and eggs. Then, using a rubber spatula, carefully fold the two mixtures together until roughly combined, trying your best not to knock any of the air out
When combined, take the bowl with the flour and cocoa, and sift that into the mixing bowl. Once again, use the spatula to carefully fold it into the mixture, until no powdery residue remains. Be careful not to over mix
Add 100g of chopped macadamia nuts and 60g of chocolate chips, and give the whole thing a brief stir, until well distributed
Transfer the whole glorious mess to the pre-prepared baking tray, and bake in the oven for approximately 30 minutes. Or, until giving the tray a shake yields no wobble whatsoever (start checking after 25 minutes) Once ready, take it out of the oven, and leave it in the tin until cooled entirely