Preheat the oven to 325°F. Line the bottom and sides of an 8×8-inch baking pan with parchment paper, leaving an overhang on two opposite sides. Lightly grease with cooking spray and set aside.

Cut butter into chunks. Combine the butter, sugar, cocoa, and salt in a medium microwave-safe bowl. Microwave for one minute intervals, stirring in between until the butter is melted and the mixture is smooth. Set the mixture aside until it cools slightly, it should be warm but not hot.

Stir in the vanilla with a wooden spoon or rubber spatula. Add the eggs one at a time stirring vigorously after each addition. When the batter looks thick, shiny, and well blended, fold in the flour just until incorporated and no streaks remain. Gently mix in the chocolate chunks. Spread the batter into the prepared pan and bake 25- 30 minutes, until just set.

Let cool on a rack for about 30 minutes. To get a clean cut, place brownies in freezer for 15-20 minutes. Lift up the ends of the parchment, place on a cutting board and cut into squares and serve.