Preheat the oven to 350°F. Line a 9x9-inch baking pan with nonstick foil or parchment paper, leaving a little overhang on two sides. In a large microwave-safe bowl, combine the butter and semisweet chocolate chips. Microwave in 30-second bursts, stirring after each burst, until the mixture is completely melted and smooth. Let it cool for a couple of minutes.

Whisk the granulated sugar, brown sugar, vanilla extract, and eggs into the melted chocolate mixture until well combined and glossy. Add the flour, salt, and cocoa powder, and stir just until no dry streaks remain. Fold in the milk chocolate chips.

Pour the batter into the prepared pan and spread it evenly. Bake for 35 to 40 minutes, checking around the 35-minute mark. For fudgier, gooier brownies, pull them out at 35 minutes. For chewier brownies, continue baking until 40 to 42 minutes. The edges should be set and the center should look slightly soft.

If serving warm, let the brownies rest for 5 to 10 minutes, then cut into 12 squares and serve with ice cream and hot fudge sauce. For the chewiest texture, cool completely in the pan before cutting. To warm individual squares, microwave them for about 20 seconds.