Step 1: Preheat your oven to 350°F (175°C). Line an 8x8 inch (20x20 cm) baking pan with parchment paper, leaving an overhang on two sides. Lightly grease the parchment paper.
Step 2: In a medium microwave-safe bowl, combine the unsalted butter, granulated sugar, and brown sugar. Microwave in 30-second intervals, stirring after each, until the butter is mostly melted and the sugars are dissolving. Alternatively, melt in a saucepan over low heat, stirring constantly until smooth.
Step 3: Remove the butter-sugar mixture from the heat. Whisk vigorously for about 1-2 minutes until slightly cooled and shiny.
Step 4: Add the eggs, one at a time, whisking well after each addition until fully incorporated. Then, stir in the vanilla extract. The mixture should be smooth and slightly thickened.
Step 5: In a separate bowl, whisk together the all-purpose flour, unsweetened cocoa powder, and salt. Ensure there are no lumps.
Step 6: Gradually add the dry ingredients to the wet ingredients, mixing with a spatula or wooden spoon until just combined. Be careful not to overmix.
Step 7: Fold in the chocolate chips (if using).
Step 8: Pour the batter into your prepared baking pan, spreading it evenly.
Step 9: Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs, but not wet batter. The edges should be set, and the center should still look a little fudgy.
Step 10: Allow the brownies to cool completely in the pan on a wire rack before lifting them out using the parchment paper overhang. Once cooled, cut into squares and enjoy.