Method

1. Preparation

   Preheat oven to 175°C. Line a 20cm by 20cm baking pan with parchment paper.

2. Chocolate

   Carefully place a medium heat proof bowl on top of a saucepan of simmering water, then add 1 cup butter into heat proof bowl. Add dark chocolate and cocoa powder. Whisk chocolate mixture occasionally until melted and smooth. Set aside chocolate mixture to cool slightly.

3. Brownie

   Place half of the blackberries into a large bowl. Add half of the raspberries, 1 cup granulated sugar, and 1 cup brown sugar. Mash berry mixture until it forms a coarse paste. Add 1 tablespoon vanilla extract, 2 teaspoons kosher or table salt, and 4 eggs. Whisk berry mixture thoroughly until smooth. Fold chocolate mixture into berry mixture, then fold in 1 ½ cups flour just until combined (try not to overmix). Then pour batter into prepared pan. Scatter remaining blackberries and raspberries over top of batter. Bake until set, about 35 to 42 minutes.

4. Serving

   Remove brownies from oven and immediately sprinkle with sea salt flakes. Slice into 24 pieces and serve.