Preheat your oven to 350°F (175°C). Grease a 9x9 inch baking pan or line it with parchment paper for easier removal.
In a large mixing bowl, combine the granulated sugar and brown sugar. Pour the melted butter over the sugars and mix until well combined.
Add the eggs and vanilla extract to the sugar mixture. Whisk until the mixture is smooth and glossy.
Sift in the cocoa powder, flour, salt, and baking powder. Stir gently with a spatula until just combined; avoid overmixing.
Fold in the chocolate chips and chopped walnuts (if using).
Pour the brownie batter into the prepared baking pan and spread it evenly.
Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted comes out with a few moist crumbs (not wet batter).
Remove from the oven and let it cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.
Once cooled, cut into squares and serve.
Notes
Serve the brownies on a rustic wooden board, dusted lightly with powdered sugar. Add a scoop of vanilla ice cream on the side for an extra indulgent treat!