Step 1: Preheat oven to 350°F (175°C). Line an 8×8 inch baking pan with parchment paper, leaving an overhang on two sides for easy removal.
Step 2: In a medium saucepan over low heat, melt the unsalted butter. Remove from heat and whisk in granulated sugar and brown sugar until well combined.
Step 3: Whisk in eggs one at a time, ensuring each is fully incorporated. Stir in vanilla extract. Whisk vigorously for 1-2 minutes until mixture lightens slightly and becomes glossy.
Step 4: In a separate bowl, whisk together all-purpose flour, unsweetened cocoa powder, and salt.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing with a spatula until just combined. Do not overmix.
Step 6: If desired, fold in chocolate chips or chopped nuts.
Step 7: Pour batter evenly into the prepared baking pan.
Step 8: Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs (not wet batter). The edges should be set, but the center may look slightly underbaked for ultimate fudginess.
Step 9: Remove from oven and let cool completely in the pan on a wire rack before cutting into squares. This step is crucial for dense, clean-cut brownies.