Cut the butter into smaller pieces so it melts down quicker. Place it in a heatproof bowl or in a medium saucepan. Add the chopped chocolate. Melt in the microwave in 20-second increments stirring after each until melted, or melt it over low heat on the stove. Cool for just 5 minutes.

Meanwhile, preheat the oven to 350°F (177°C). Line the bottom and sides of a 9-inch square baking pan with parchment paper. Set aside.

Whisk the granulated and brown sugars into the cooled chocolate/butter mixture. Whisk in the eggs, and then the vanilla. Whisk in the flour, cocoa powder, and salt until no pockets of dry ingredients remain. Once combined, fold in the chocolate chips. The batter is semi-thick.

Pour and spread batter evenly into the prepared baking pan. Bake for 32-35 minutes, or until the top appears set and a toothpick inserted in the center comes out with a few moist crumbs. Check them at 30 minutes.

Place the pan on a wire rack and cool brownies for 1 to 2 hours. Once cooled, lift the brownies out of the pan using the parchment paper on the sides. Set aside as you make the frosting.

With a handheld or stand mixer fitted with a paddle or whisk attachment, beat the butter on medium speed until creamy, about 2 minutes. Add confectioners’ sugar, cocoa powder, heavy cream/milk, vanilla extract, and salt. Beat on low speed for 30 seconds, then increase to high speed and beat for 1 full minute. Taste and adjust salt if needed.

Spread over mostly cooled brownies. Cut into squares for serving.