Line an 8" square pan with aluminum foil or parchment paper. Spray with cooking spray. Preheat oven to 350 degrees.
Place the butter and chopped chocolate in a large microwave safe bowl. Melt at half power, stirring every 30 seconds.
Stir in the sugar, then the eggs and vanilla. Add the flour and salt, stir just until there are no streaks. Fold in the chocolate chips.
Spread the brownie batter in the prepared pan. Bake at 350° for 33-38 minutes, or until a toothpick comes out with moist crumbs, but no wet batter.
Place the pan on a wire rack and let the brownies cool completely.
For frosting, beat the soft butter, cocoa, salt, powdered sugar, vanilla, and 3 tablespoons of the milk with a hand mixer. Add more milk as needed to reach a spreadable consistency.
Spread the frosting over the cooled brownies. Lift the brownies out of the pan using the foil. Place on a cutting board, cut into squares, and serve.