Preheat the oven to 150C (conventional - top and bottom heat). Line a lamington pan (we use a 38cm x 26cm) rectangular cake tin with baking paper.
Melt butter and chocolate in a medium size pot on induction, power level 3 stirring occasionally. Mix to incorporate.
In the meantime, in a stand mixer fitted with the whisk attachment and a medium bowl, whisk eggs, sugar, vanilla and salt until pale and fluffy.
Sift cocoa powder and flour into a separate bowl.
With the machine still operating on slow speed, add chocolate mixture into egg mixture then incorporate the flour and cocoa powder mixture.
Pour it into the prepared cake pan. Bake on the middle shelf for 50 minutes.
Allow to cool completely then transfer to the fridge. It is easier to portion when the brownie is fridge cold and it will also give you a cleaner cut.
Cut into squares.