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 salt in a medium bowl. Set aside.
03 - In a heatproof bowl set over simmering water, melt the bittersweet chocolate and butter together, 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 well combined and slightly thickened.
05 - Stir the melted chocolate mixture into the sugar and egg mixture until fully blended.
06 - Gently fold the dry ingredients into the wet mixture just until no streaks remain, taking care not to overmix.
07 - Fold in the chopped walnuts evenly throughout the batter.
08 - Pour the batter into the prepared baking pan and smooth the surface with a spatula.
09 - Bake for 28 to 32 minutes, or until a toothpick inserted into the center comes out with moist crumbs attached.
10 - Allow the brownies to cool completely in the pan on a wire rack before lifting out using the parchment overhang and slicing into squares.