01 - Preheat 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 salt in a medium bowl. Set aside.
03 - Melt bittersweet chocolate and butter together in a heatproof bowl over simmering water, stirring until smooth. Remove from heat and cool slightly.
04 - Whisk granulated sugar, brown sugar, eggs, and vanilla in a large bowl until well combined and slightly thickened.
05 - Stir the melted chocolate and butter mixture into the egg mixture until fully incorporated.
06 - Gently fold in the dry ingredient mixture just until no streaks remain; avoid overmixing.
07 - Fold the chopped walnuts into the batter evenly.
08 - Pour the batter into the prepared pan and smooth the surface.
09 - Bake for 28 to 32 minutes, or until a toothpick inserted in the center comes out with moist crumbs attached.
10 - Allow brownies to cool completely in the pan on a wire rack before lifting out using the parchment overhang and slicing into 16 squares.