Preheat oven to 180ºC (160 fan forced). Line a 9x9 inch brownie pan with baking paper.
In a bowl, add butter and chocolate. Microwave at intervals of 30 seconds for approximately two minutes, stirring in between. Mixture should be smooth, shiny and silky. Allow to cool slightly.
In a large bowl, add sugar, flour, cocoa powder and salt and stir to combine. Add melted chocolate and butter, eggs and vanilla and mix gently to form a batter. Stir through blueberries.
Pour batter into prepared tin and bake for 25-30 minutes. The centre should be soft but not jiggling.
Allow to cool completely before slicing. Sprinkle with sea salt flakes to serve.
Notes
Substitute the castor sugar with 3/4 cup stevia powder if you so choose.