1. Add golden syrup, dark chocolate and butter to a bowl set over simmering water, stir until fully melted.
2. Whisk together sugars and eggs until pale and thick.
3. Drizzle in melted dark chocolate and fold through so as not to knock out the air from the eggs.
4. Sift into the batter the flour, cocoa powder and salt and fold through until combined.
5. Add in chopped white chocolate and milk chocolate chips and fold through.
6. Pour into a lined baking pan and bake for 20-25 minutes. Remove from pan and sprinkle over flaked salt. Slice into squares.