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 set over simmering water, melt chocolate and butter together, stirring until smooth. Remove from heat and cool slightly.
03 - Whisk granulated and brown sugars into the melted chocolate mixture until fully incorporated.
04 - Add eggs one at a time, whisking well after each addition. Stir in vanilla extract.
05 - Sift flour, cocoa powder, and fine sea salt together, then gently fold into the wet mixture with a spatula until just combined; avoid overmixing.
06 - Pour batter into prepared pan, smooth the surface, and evenly sprinkle with flaky sea salt.
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. Lift out using parchment overhang and cut into 16 squares.