Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking pan or line it with parchment paper.
In a medium saucepan, melt the butter over medium heat. Remove from heat and stir in the granulated sugar and brown sugar until well combined.
Allow the butter and sugar mixture to cool slightly, then add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
In a separate bowl, sift together the flour, cocoa powder, salt, and baking powder. Gradually add the dry ingredients to the wet mixture, stirring until just combined.
Gently fold in the chopped walnuts until evenly distributed throughout the batter.
Pour the batter into the prepared baking pan and spread it out evenly. Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the centre comes out with a few moist crumbs.
Allow the brownies to cool completely in the pan before cutting them into squares. Enjoy the rich, chocolaty goodness with the added crunch of walnuts!