Preheat your oven to 350°F (175°C). To prepare your baking dish, either grease a 9x9 inch pan with butter or cooking spray or line it with parchment paper for easy removal later.
In a large mixing bowl, combine the granulated sugar and packed brown sugar. Pour the melted butter over the sugar mixture and stir thoroughly until it is smooth and evenly blended.
Add the eggs and pure vanilla extract to the bowl. Using a whisk, mix until the batter becomes smooth and glossy, which will ensure your brownies are rich and fudgy.
Sift the cocoa powder, all-purpose flour, fine salt, and baking powder directly into the bowl. Gently fold the ingredients together with a spatula, mixing until just combined. Be careful not to overmix, as this can lead to a cakey texture.
Carefully fold in the semi-sweet chocolate chips and the chopped walnuts (if using), ensuring they are evenly distributed throughout the batter.
Pour the brownie batter into your prepared baking pan and spread it out evenly with a spatula to create a uniform layer.
Place the pan in your preheated oven and bake for 20-25 minutes. To check for doneness, insert a toothpick into the center; it should come out with a few moist crumbs, indicating the brownies are perfectly baked. If you see wet batter, bake for an additional few minutes.
Once baked, remove the brownies from the oven and allow them to cool in the pan for about 10 minutes. Then transfer them to a wire rack to cool completely, which helps maintain their fudgy texture.
After cooling, cut the brownies into squares of your desired size and serve immediately for optimum indulgence.
Notes
For an attractive display, serve the brownies on a rustic wooden board, lightly dusted with powdered sugar. For a decadent twist, add a scoop of creamy vanilla ice cream alongside each brownie.