Preheat oven to 350 degrees Fahrenheit. Line the baking pan with parchment paper so that at least one inch of parchment paper hangs over the edge of the pan. Set aside.
Roughly chop chocolate bar and cut butter into chunks then place both into a heat-safe bowl and microwave in 30-second increments until melted (typically takes between one minute and 90 seconds total). Stir together until uniform.
6 ounces dark chocolate, ½ cup unsalted butter
Mix eggs, granulated sugar, brown sugar, and vanilla extract into chocolate and butter mixture until combined.
3 eggs, ⅔ cup granulated sugar, ½ cup brown sugar, 1 teaspoon vanilla bean paste
In another bowl, whisk together flour, cocoa powder, and fine sea salt then dredge the walnuts and pecans in this mixture until coated.
½ cup all-purpose flour, 3 tablespoons cocoa powder, ¾ teaspoon fine sea salt, ½ cup walnuts, ½ cup pecans
Add the flour mixture to the chocolate mixture and stir until just combined, then pour the batter into the prepared baking dish, spreading it evenly across the pan.
Place pan into preheated oven and bake for 28-32 minutes or until baked through. *When done, a toothpick inserted into the center of the brownie will come out with moist fudgy brownie crumbs clinging to it but not wet and runny streaks of batter. If it comes out completely clean, the brownies are overbaked.
Let brownies cool for at least 30 minutes before cutting. Use parchment paper to lift brownies out of pan then cut into 16 equal pieces.