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