Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan with butter or cooking spray, or line it with parchment paper, leaving some overhang for easy brownie removal.
In a large saucepan, melt the butter over medium heat, stirring gently. Once melted, remove from the heat and stir in granulated sugar until the mixture is smooth and well-combined.
Allow the buttery mixture to cool slightly. Then, one by one, whisk in each egg, ensuring that the mixture is smooth and homogenous before adding the next. Stir in the vanilla extract until fully incorporated.
In a separate medium bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking powder, and fine sea salt until there are no lumps.
Gradually add the dry ingredients to the wet mixture, using a spatula to fold gently until just combined. Be mindful not to overmix to maintain a fudgy texture.
If desired, gently fold in the chocolate chips and chopped nuts, distributing them evenly throughout the batter for added richness and crunch.
Pour the rich brownie batter into your prepared baking pan, using a spatula to spread it evenly across the surface.
Bake in the preheated oven for 25-30 minutes. To check for doneness, insert a toothpick in the center; it should come out with a few moist crumbs attached, indicating a perfect fudgy brownie (not wet batter).
Once baked, allow the brownies to cool completely in the pan set on a wire rack before slicing into squares.
Notes
For an eye-catching presentation, drizzle warm chocolate sauce over the brownies or lightly dust with powdered sugar. Serve alongside a scoop of velvety vanilla ice cream.