1
Add unsalted butter and dark chocolate to a bowl.
2
Take a small saucepan and fill it approximately one-fourth of the way with hot water. Place the bowl on top of the saucepan, ensuring that it rests on the edges of the pan and does not make contact with the water. Heat the pan on low until the butter and chocolate have melted, occasionally stirring to combine them.
3
Take the bowl off the pan and allow the melted mixture to cool down until it reaches room temperature.
4
As the chocolate cools, preheat the oven to 180°C.
5
Cut a square of kitchen foil or non-stick baking parchment to line the base of a shallow 20cm square tin.
6
Crack the large eggs into a large bowl and add the golden caster sugar. Whisk them together until they become thick and creamy mousse.
7
Drizzle the cooled chocolate mixture onto the eggy mousse and use a spatula to carefully mix them together. Fold the ingredients until they have a mottled dark brown color. The key is to combine them without deflating the mixture, so proceed with a gentle and gradual approach.
8
Pour the plain flour and cocoa powder into a sieve positioned over the bowl with the eggy chocolate mixture. Shake the sieve back and forth to evenly spread the ingredients on top and carefully fold them in.
9
Lastly, add the white and milk chocolate chunks and mix until they are distributed evenly.
10
Transfer the mixture into the prepped tin and carefully spread the mixture into the corners of the tin.
11
Place the tin in the oven and bake for 25 minutes. Keep baking until the top develops a shiny, papery crust and the edges start to detach from the tin.
12
Remove the tin from the oven and let it cool down completely.
13
Divide it into quarters, then divide each quarter into four square pieces.