Preheat your oven to 350°F (175°C). Line an 8x8-inch (20x20 cm) baking pan with parchment paper, leaving an overhang on the sides to lift the brownies out easily.
In a medium saucepan over medium heat, melt the butter. Once melted, remove from heat and stir in both granulated and brown sugars, then add the cocoa powder and salt, mixing until well combined.
Add the vanilla extract and mix. Then, add eggs one at a time, fully incorporating each egg before adding the next. After adding eggs, beat the mixture vigorously for about 1 minute until it becomes thick and shiny.
Gently fold in the flour until just combined, being careful not to overmix. If using, fold in the chocolate chips or chunks.
Pour the batter into the prepared pan and smooth the top with a spatula. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Avoid overbaking to maintain a fudgy texture.
Allow the brownies to cool completely in the pan set on a wire rack.