01 - Preheat the oven to 350°F. Line an 8x8 inch baking pan with parchment paper, leaving an overhang for easy removal.
02 - In a large bowl, whisk together the melted butter, granulated sugar, and brown sugar until combined and glossy.
03 - Add eggs one at a time, mixing well after each addition. Stir in vanilla extract.
04 - Sift in the flour, cocoa powder, and salt. Gently fold until just combined; do not overmix.
05 - Fold in the chopped chocolate.
06 - Pour the batter into the prepared pan, smoothing the top.
07 - Bake for 28–32 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
08 - Cool completely in the pan on a wire rack before frosting.
09 - In a medium saucepan, melt the butter over low heat. Stir in cocoa powder until smooth.
10 - Remove from heat. Add milk, vanilla, and salt, stirring to combine.
11 - Gradually whisk in the powdered sugar until a thick, spreadable frosting forms. Add a little more milk if needed.
12 - Spread the frosting evenly over the cooled brownies. Let set for 10–15 minutes before cutting into squares.