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, if you’re using a saucepan, 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 (not wet) crumbs. All ovens are different, so keep an eye on your brownies. Check them at 30 minutes. Mine usually take 32 minutes. Tip: Err on the side of underbaking, because brownies continue to set as they cool.

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. Beat in another pinch of salt if desired.

Spread over mostly cooled brownies (it’s ok if the brownies are still a bit warm). I usually use an offset spatula to spread the frosting. Cut into squares. For neat squares, I use a very long and sharp knife and wipe it clean with a paper towel after each cut.

Unfrosted brownies stay fresh in an airtight container at room temperature for 1 week. Frosted brownies stay fresh in an airtight container at room temperature for 2 days or in the refrigerator for up to 1 week.