Step 1: Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper.
Step 2: In a heatproof bowl set over a saucepan of simmering water (or in the microwave in 30-second intervals), melt the unsalted butter and chopped dark chocolate or chocolate chips together. Stir until smooth and completely melted. Remove from heat and let cool slightly.
Step 3: In a large mixing bowl, whisk together the granulated sugar, brown sugar, and eggs until light in color and slightly thickened, about 2-3 minutes. Whisk in the vanilla extract.
Step 4: Gradually pour the slightly cooled chocolate-butter mixture into the egg-sugar mixture, whisking constantly until well combined and smooth.
Step 5: In a separate medium bowl, whisk together the all-purpose flour, Dutch-processed cocoa powder, baking powder, and salt.
Step 6: Add the dry ingredients to the wet ingredients, mixing with a spatula or wooden spoon until just combined. Do not overmix.
Step 7: Gently fold in the additional chocolate chips or chunks.
Step 8: Drop rounded tablespoons of dough onto the prepared baking sheets, spacing them about 2 inches apart.
Step 9: Bake for 10-12 minutes, or until the edges are set but the centers still look slightly soft and gooey. Do not overbake.
Step 10: Remove from the oven and let the brownie cookies cool on the baking sheets for 5-10 minutes before transferring them to a wire rack to cool completely.