01 - Preheat oven to 350°F (175°C). Line a 9 x 9 inch baking pan with parchment paper, ensuring an overhang for easy removal.
02 - In a heatproof bowl positioned over a saucepan of simmering water, melt the chopped dark chocolate and cubed unsalted butter together, stirring until completely smooth. Remove from heat and allow the mixture to cool slightly.
03 - Whisk in both the granulated sugar and light brown sugar into the chocolate mixture until it becomes glossy and well combined.
04 - Add the large eggs one at a time, whisking vigorously after each addition until fully incorporated. Stir in the pure vanilla extract and fine sea salt.
05 - Sift the all-purpose flour and unsweetened cocoa powder directly into the wet ingredients. Gently fold the dry ingredients into the wet mixture with a spatula until just combined; take care not to overmix.
06 - Carefully fold in the roughly chopped walnuts, reserving a small handful to sprinkle on top before baking.
07 - Pour the prepared brownie batter evenly into the lined baking pan and smooth the top surface with your spatula. Sprinkle with the reserved walnuts.
08 - Bake for 28–32 minutes, or until a wooden toothpick inserted into the center comes out with a few moist crumbs. Avoid overbaking to ensure a rich, fudgy texture.
09 - Allow the brownies to cool completely in the pan on a wire rack before using the parchment paper overhang to lift them out. Cut into squares and serve.