Heat the oven to 350℉.

Coarsely chop 6 ounces bittersweet chocolate if not using chocolate chips. Place the chocolate and 1 ½ sticks cut unsalted butter in a large microwave-safe bowl. Microwave on HIGH in 20-second increments, stirring between each, until the chocolate is melted and the mixture is smooth, about 2 minutes total. (Alternatively, melt together in a small saucepan over low heat.)

Whisk in ¼ cup natural cocoa powder until smooth and combined. Let sit for 5 minutes so the cocoa powder can bloom. Meanwhile, coat a 9x13-inch baking pan, preferably metal, with cooking spray. Line the bottom and two long sides of the pan with a sheet of parchment paper that extends a few inches over the sides to form a sling.

Whisk 2 cups granulated sugar into the butter mixture until combined. Whisk in 4 large eggs one at a time, whisking well in between each addition. Continue whisking until the mixture is smooth and glossy. Whisk in 1 tablespoon vanilla extract and ½ teaspoon kosher salt until combined.

Add 1 ½ cups all-purpose flour and gently fold with a flexible spatula until incorporated and no streaks of flour remain; do not overmix. Transfer to the baking pan and spread into an even layer.

Bake until a tester inserted in the center comes out clean, 27 to 30 minutes. Let cool for 15 minutes. Meanwhile, prepare the frosting.

Beat 4 tablespoons room-temperature unsalted butter and ¼ teaspoon kosher salt in a large bowl with an electric hand mixer on medium speed until lightened in color and fluffy, about 2 minutes. Scrape down the sides of the bowl with a flexible spatula.

With the mixer on low speed, slowly beat in 1 ¾ cups powdered sugar, ¼ cup cocoa powder, ¼ cup whole milk, and 1 teaspoon vanilla extract. Gradually increase the speed to medium-high and beat, scraping the bowl halfway through, until well-combined, smooth, and fluffy, 2 to 3 minutes total.

Working quickly, spread the frosting evenly over the warm brownies (it will start to melt). Let sit until cooled completely and the frosting sets, about 2 hours.

Grasping the excess parchment, lift the brownie slab out of the pan and place on a cutting board. Cut into 24 pieces.