01 - Preheat oven to 350°F. Grease a 10-inch cast-iron skillet with butter.
02 - In a microwave-safe bowl or double boiler, melt unsalted butter and 4 ounces chopped dark chocolate together, stirring until smooth. Allow mixture to cool slightly.
03 - Whisk granulated sugar into the melted chocolate mixture until blended. Add eggs one at a time, mixing well after each addition. Stir in vanilla extract. In a separate bowl, whisk together all-purpose flour, unsweetened cocoa powder, and salt. Gently fold dry ingredients into chocolate mixture until just combined. Stir in chocolate chips.
04 - Spread half of the brownie batter evenly in the prepared skillet. Arrange dark chocolate chunks in the center and drizzle with heavy cream. Gently spread the remaining batter over the top to cover the filling.
05 - Bake for 22 to 25 minutes, until edges are set but the center still jiggles slightly when shaken. The top should appear cracked and the center should remain slightly underbaked for a fudgy texture.
06 - Allow brownie to cool in the skillet for 5 minutes. Dust surface with powdered sugar. Serve warm directly from the skillet, accompanied by vanilla ice cream, hot fudge sauce, and fresh berries if desired.