Cut unsalted butter into small cubes and tip into a medium bowl.
Break dark chocolate into small pieces and drop into the bowl.
Fill a small saucepan about a quarter full with hot water, then sit the bowl on top so it rests on the rim of the pan, not touching the water. Put over a low heat until the butter and chocolate have melted, stirring occasionally to mix them.
Remove the bowl from the pan.
Using a shallow 20cm square tin, cut out a square of non-stick baking parchment to line the base.
Tip plain flour and cocoa powder into a sieve held over a medium bowl.
Tap and shake the sieve so they run through together and you get rid of any lumps.
Chop milk chocolate into chunks on a board.
Break eggs into a large bowl and tip in golden caster sugar and whisk until double in volume.
Pour the cooled chocolate mixture over the eggy mousse, then gently fold together with a rubber spatula.
Hold the sieve over the bowl of eggy chocolate mixture and resift the cocoa and flour mixture, shaking the sieve from side to side, to cover the top evenly.
Gently fold in this powder. The mixture will look dry and dusty at first, and a bit unpromising, but if you keep going very gently and patiently, it will end up looking gungy and fudgy. You don’t want to overdo this mixing.
Finally, stir in the white and milk chocolate chunks until they’re dotted throughout.
Pour the mixture into the prepared tin, scraping every bit out of the bowl with the spatula. Gently ease the mixture into the corners of the tin and paddle the spatula from side to side across the top to level it.
Put in the oven and set your timer for 25 minutes.
In a medium pot, cook apricots, in water and sugar for 10 minutes. Then place it in a blender and blend until smooth.
Serve brownies with apricot puree and toasted hazelnuts.