01 - Preheat your oven to 350°F. Line a 9 x 9 inch baking pan with parchment paper, ensuring an overhang on the sides for easy removal of the brownies.
02 - In a heatproof bowl positioned over a saucepan of gently simmering water, melt the chopped dark chocolate and cubed unsalted butter together. Stir continuously until the mixture is smooth and fully combined. Remove the bowl from the heat and allow the mixture to cool slightly.
03 - Whisk both the granulated sugar and light brown sugar into the cooled chocolate mixture until it becomes glossy and thoroughly combined.
04 - Incorporate the eggs one at a time, whisking vigorously after each addition until fully integrated. 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. Using a spatula, gently fold the dry ingredients into the batter until just combined, being careful not to overmix.
06 - Fold in the majority of the chopped walnuts, reserving a small handful to sprinkle over the top of the batter before baking.
07 - Pour the brownie batter into the prepared baking pan and spread it evenly. Sprinkle the reserved walnuts over the surface of the batter. Bake for 28 to 32 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Avoid overbaking to ensure a fudgy texture.
08 - Allow the brownies to cool completely in the pan before lifting them out using the parchment paper overhang. Once cooled, cut them into squares for serving.