01 - Preheat the oven to 350°F. Line an 8x8 inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a heatproof bowl over simmering water, melt chocolate and butter together, stirring until smooth. Remove from heat and let cool slightly.
03 - Whisk granulated sugar and light brown sugar into the melted chocolate until well blended.
04 - Add eggs one at a time, whisking thoroughly after each addition; then stir in vanilla extract.
05 - Sift flour, cocoa powder, and fine sea salt together. Gently fold into the wet mixture with a spatula until just combined, avoiding overmixing.
06 - Pour batter into prepared pan and smooth the surface. Sprinkle flaky sea salt evenly on top.
07 - Bake for 28–32 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
08 - Allow to cool completely in the pan on a wire rack. Use parchment overhang to lift brownies out and cut into 16 squares.