Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan, or line it with parchment paper, leaving an overhang on two sides.
Step 2: In a medium microwave-safe bowl, combine the unsalted butter and 1 cup of semisweet chocolate chips. Microwave in 30-second intervals, stirring after each, until mostly melted and smooth. Let it cool slightly.
Step 3: In a large bowl, whisk together the granulated sugar and brown sugar. Add the cooled melted butter-chocolate mixture, vanilla extract, and eggs (one at a time), mixing well after each addition until just combined and smooth. Be careful not to overmix.
Step 4: In a separate large bowl, whisk together the all-purpose flour, unsweetened natural cocoa powder, baking powder, and salt.
Step 5: Gradually add the dry ingredient mixture to the wet ingredient mixture, stirring only until just combined. Overmixing can lead to tough brownies. If desired, gently fold in the additional 1/2 cup of chocolate chips.
Step 6: Pour the brownie batter into your prepared 9x13 inch baking pan, spreading it evenly with a spatula.
Step 7: Bake for 25-30 minutes, or until a wooden skewer inserted into the center comes out with moist crumbs, but not wet batter. The edges should be set, but the center might still look a little soft.
Step 8: Let the brownies cool completely in the pan on a wire rack before cutting into squares. This is crucial for achieving a fudgy texture. Once cooled, you can lift them out using the parchment paper overhang for perfect slices.