Step 1: Preheat your oven to 350°F (175°C). This ensures that your brownies bake evenly.
Step 2: Line your 9x9-inch baking pan with parchment paper, leaving some overhang on the sides. This will help you lift the brownies out later for easy slicing.
Step 3: In a double boiler or a microwave-safe bowl, combine the chopped semisweet, bittersweet, and unsweetened chocolate with the butter. Stir occasionally until melted and smooth.
Step 4: Once the chocolate and butter are melted, whisk in the cocoa powder until fully incorporated, creating a rich chocolate base.
Step 5: In a separate bowl, whisk together the eggs, granulated sugar, and vanilla extract until light and slightly frothy.
Step 6: Gradually add the melted chocolate mixture to the egg and sugar blend, stirring until well combined.
Step 7: Sift the flour and salt over the chocolate mixture, then gently fold in using a rubber spatula.
Step 8: Pour the brownie batter into your prepared pan, smoothing the top. Bake for 25-30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Step 9: Allow the brownies to cool in the pan for about 10 minutes before lifting them out using the parchment paper. Let them cool completely on a wire rack before slicing into squares.