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 any exposed pan sides.
Step 2: In a large, heatproof bowl, combine the chopped dark chocolate and melted butter. Melt gently over a double boiler or in the microwave in 30-second intervals, stirring until smooth. Let cool slightly.
Step 3: In a separate medium bowl, whisk together the granulated sugar, light brown sugar, eggs, and vanilla extract until well combined and slightly lightened.
Step 4: Pour the slightly cooled chocolate-butter mixture into the egg-sugar mixture. Whisk until fully incorporated.
Step 5: In a small bowl, whisk together the all-purpose flour, Dutch-processed cocoa powder, and salt.
Step 6: Gradually add the dry ingredients to the wet ingredients. Fold gently with a spatula until just combined. Do not overmix.
Step 7: If using, gently fold in any optional mix-ins like chocolate chips or nuts.
Step 8: Pour the batter into the prepared baking pan and spread evenly. Tap the pan gently on the counter to release air bubbles.
Step 9: Bake for 28-35 minutes, or until the edges are set and a toothpick inserted into the center comes out with moist crumbs (not wet batter). The center should still look slightly gooey.
Step 10: Remove from the oven and let cool completely in the pan on a wire rack before lifting out and cutting into 16 squares. Store in an airtight container at room temperature.