01 - Preheat the oven to 350°F. Line an 8x8 inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - Set a heatproof bowl over simmering water and melt together the chopped chocolate and butter, stirring until smooth. Remove from heat and allow to cool slightly.
03 - Whisk the granulated sugar and brown sugar into the melted chocolate and butter until fully incorporated.
04 - Add the eggs one at a time, whisking thoroughly after each. Stir in the vanilla extract until blended.
05 - Sift the flour, cocoa powder, and fine sea salt together and gently fold into the wet mixture with a spatula until just combined, avoiding overmixing.
06 - Pour the batter into the prepared pan, smoothing the surface evenly. Sprinkle flaky sea salt on top.
07 - Bake for 28 to 32 minutes, 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 the parchment overhang to lift out and cut into 16 squares.