01 - Preheat oven to 350°F and line an 8x8 inch baking pan with parchment paper, leaving overhang for easy removal.
02 - Whisk melted butter, granulated sugar, and light brown sugar in a large bowl until glossy and combined.
03 - Add eggs one at a time, mixing well after each addition, then stir in vanilla extract.
04 - Sift together flour, cocoa powder, and salt; gently fold into wet mixture until just combined to avoid overmixing.
05 - Incorporate chopped bittersweet chocolate evenly into the batter.
06 - Pour batter into prepared pan, smooth top, and bake for 28 to 32 minutes until a toothpick inserted near the center comes out with moist crumbs.
07 - Allow brownies to cool completely in the pan set on a wire rack before applying frosting.
08 - Melt butter over low heat in a saucepan, stir in cocoa powder until smooth, then remove from heat.
09 - Add milk, vanilla, and salt to the cocoa mixture and combine, then gradually whisk in powdered sugar until a thick, spreadable consistency is achieved; add milk if needed to adjust.
10 - Spread frosting evenly over cooled brownies and allow to set for 10 to 15 minutes before slicing into 16 squares.