01 - Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, ensuring an overhang on the sides for easy removal of the baked brownies.
02 - In a spacious mixing bowl, vigorously whisk together the melted butter, granulated sugar, and light brown sugar until the mixture is thoroughly combined and exhibits a glossy texture.
03 - Integrate the eggs into the sugar mixture one at a time, ensuring thorough mixing after each addition. Stir in the vanilla extract until fully combined.
04 - Sift the all-purpose flour, cocoa powder, and salt directly into the wet ingredients. Gently fold these dry components until they are just combined, taking care not to overmix the batter.
05 - Carefully fold the chopped bittersweet chocolate into the prepared brownie batter.
06 - Transfer the brownie batter into the prepared baking pan, spreading it evenly across the bottom. Bake for approximately 28 to 32 minutes, or until a wooden toothpick inserted into the center emerges with a few moist crumbs attached.
07 - Allow the baked brownies to cool completely in the pan on a wire rack before proceeding with the frosting application.
08 - For the fudge frosting, melt the unsalted butter in a medium saucepan over low heat. Once melted, stir in the cocoa powder until the mixture is smooth.
09 - Remove the saucepan from the heat. Incorporate the milk, vanilla extract, and a pinch of salt, stirring until well combined. Gradually whisk in the sifted powdered sugar until a thick, smooth, and spreadable frosting consistency is achieved. If necessary, add a small additional amount of milk to reach the desired texture.
10 - Evenly spread the prepared fudge frosting over the completely cooled brownies. Allow the frosting to set for 10 to 15 minutes before cutting the brownies into individual squares for serving.