ANZAC BISCUIT BASE

1. Preheat the oven to 325°f / 160°c Grease and line a 9x13" (20cm x 30cm) baking pan with parchment paper, leaving some overhanging to help remove the brownie later.
2. Place the butter in a medium saucepan, and place over medium heat. Cook until the butter has melted, and then continue to cook, swirling the pan often, until the butter foams and turns golden brown and nutty - this should take 3-4 minutes. Transfer to a bowl and allow to cool slightly.
3. Weigh out 115g (see Notes section below) of the brown butter and place into a small saucepan. 
4. Add the Golden Syrup and water, and place over a low heat, stirring frequently, until smooth and melted together.
5. While the wet ingredients are heating up, in a large bowl, combine the flour, coconut, oats, brown sugar, caster sugar, and salt. Mix until full incorporated.
6. Once the butter and golden syrup is smooth and combined, remove from the heat and add the baking soda, mixing well to incorporate (it will foam up a little). Pour into the dry ingredients and stir with a spatula until well combined.
7. Transfer the base mixture to the prepared pan, and smooth out (it will be quite thin). I like to use an offset spatula or my hands. Make sure that it is evenly spread out. 
8. Bake the Anzac biscuit base for 20 minutes, until golden brown. Remove from the oven and allow to cool completely (this step can be done a day ahead and stored covered with plastic wrap).

BROWNIE TOPPING

1. Preheat the oven to 350°f/180°c.
2. In a heatproof bowl over a pan of simmering water (make sure the bowl does not touch the water), combine the dark chocolate and butter. Heat, stirring frequently, until the chocolate and butter are melted and the mixture is smooth. Alternatively you can do this step in the microwave - microwave for 30 second increments, stirring well after each until the mixture is smooth and combined. 
3. Sift the cocoa powder over top of the chocolate and butter mixture and stir until well combined.
4. In a large bowl, combine the eggs, caster sugar, and brown sugar. Mix on medium to high speed with an electric handheld mixer for 2-3 minutes until the mixture has lightened and increased in volume. This step can also be done in a stand mixer using the whisk attachment.
5. Add the chocolate mixture and mix to combine. Add the salt and vanilla and mix until incorporated. 
6. Add the flour and incorporate by hand until the mixture is smooth and does not have any lumps. 
7. Pour the batter over the cooled Anzac base, then leave to sit for a few minutes, then bang on the counter to help remove any big bubbles. 
8. Bake the brownie slice for 20 to 25 minutes, until the top and edges are set. 
9. Remove from the oven and allow to cool, then transfer to the fridge and allow to chill completely. 
10. Remove from the pan using the overhanging parchment, and slice into pieces using a sharp knife, wiping between cuts. 
11. Store leftovers in an airtight container in the fridge for up to a week.