Preheat the oven to 350 degrees Fahrenheit (175 degrees Celsius) and grease a 9-inch cast-iron skillet.

Combine the butter and chopped semi-sweet chocolate in a saucepan over low heat, stirring continuously until melted and smooth.

Remove the saucepan from the heat and whisk in the granulated sugar until fully incorporated.

Add the eggs one at a time, whisking vigorously after each addition to ensure a glossy texture.

Whisk in the vanilla extract and salt.

Gently fold in the sifted cocoa powder until no dry streaks remain.

Stir in the chocolate chips and pour the batter into the prepared skillet.

Bake for 22 to 25 minutes, or until the edges are firm but the center remains slightly jiggly.

Allow the brownie to cool in the skillet for at least 10 minutes before serving.