01 - Preheat the oven to 350°F. Grease and line an 8-inch square baking pan with parchment paper, leaving an overhang for easy removal.
02 - Whisk together flour, cocoa powder, baking powder, and fine sea salt in a medium bowl; set aside.
03 - Place chocolate and butter in a heatproof bowl over simmering water, stirring until smooth. Remove from heat and allow to cool slightly.
04 - In a large bowl, whisk granulated sugar, brown sugar, eggs, and vanilla extract until fully blended and slightly thickened.
05 - Stir the melted chocolate and butter into the egg mixture until evenly combined.
06 - Gently fold the dry ingredients into the chocolate mixture until no streaks remain, being careful not to overmix.
07 - Fold in the chopped walnuts evenly throughout the batter.
08 - Pour the batter into the prepared pan and smooth the surface with a spatula.
09 - Bake in the preheated oven for 28 to 32 minutes, or until a toothpick inserted in the center comes out with moist crumbs attached.
10 - Cool completely in the pan on a wire rack, then lift out using the parchment overhang and slice into 16 squares.