Method
Prep:15min  ›  Cook:20min  ›  Ready in:35min 

1. Prepare a large baking tray by buttering it and lining it with parchment, then buttering the parchment. Spread 1/3 of the finely chopped macadamina nuts over the tray in an even layer.
2. Add butter and vanilla to a large, heavy pan over medium heat. Once the butter has melted, add sugar, brown sugar and salt. Stir slowly but constantly until the mixture begins to boil.
3. Keeping the heat on medium, stir mixture slowly and occasionally to avoid scorching. Using a sugar thermometer to monitor the temperature, remove from heat immediately once the temperature reaches 150 degrees C. The mixture will be a dark amber colour and will just have started to smoke.
4. Carefully pour mixture in a thin layer over the chopped nuts; let cool for 2-3 minutes. Sprinkle with a few pinches of coarse sea salt, then sprinkle with chopped chocolate. Wait 2-3 minutes, then spread the melted chocolate in a thin layer over the candy. Sprinkle with remaining chopped nuts, then press into the chocolate with the back of a greased spoon.

Cook's notes

Patience is important - do not raise the temperature higher than medium and only stir occasionally once the mixture starts to boil to ensure that the butter and sugar do not separate.
A sugar thermometer isn't required but is extremely helpful in ensuring reliable results.
This is also great with dark or milk chocolate and using almonds, hazelnuts or pecans.