01 - Preheat oven to 350°F. Line an 8x8 inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - Combine chocolate and butter in a heatproof bowl over simmering water, stirring until smooth. Remove from heat and cool slightly.
03 - Whisk granulated sugar and brown sugar into the melted chocolate mixture until uniformly blended.
04 - Add eggs one at a time, whisking thoroughly after each. Stir in vanilla extract.
05 - Sift flour, cocoa powder, and fine sea salt into wet mixture. Gently fold with a spatula until just combined, avoiding overmixing.
06 - Pour batter into prepared pan, smoothing the surface. Evenly sprinkle flaky sea salt over the top.
07 - Bake for 28 to 32 minutes until a toothpick inserted in the center yields moist crumbs.
08 - Cool completely in pan on a wire rack. Lift out using parchment overhang and cut into 16 squares.