Preheat oven to 325 degrees F. Prepare the chocolate brownie layer. Coat a 9 by 1 1/2-inch cake pan with 1 teaspoon of butter. Flour the pan with 1 teaspoon of flour, shaking out the excess.

Sift together 1/4 cup flour, 2 tablespoons cocoa, 1 teaspoon baking powder, and 1/2 teaspoon salt onto waxed paper. Set aside.

Heat 1-inch of water in the bottom half of a double boiler over medium heat. Place 3 ounces unsweetened chocolate, 4 tablespoons butter, and 2 ounces semisweet chocolate in the top half of the double boiler tightly cover top with film wrap. Heat for 4 1/2 to 5 minutes, remove from the heat, and stir until smooth.

Place 3 eggs, 1 cup sugar, and 1 teaspoon vanilla in the bowl of an electric mixer fitted with a balloon whip. Whisk on high speed until slightly thickened, about 1 1/2 minutes. Add the melted chocolate mixture into the egg mixture and whisk on medium for 30 seconds. Add the sifted ingredients, whisk on low for 10 seconds, then on medium for 10 seconds. Add the sour cream and whisk on medium for 5 seconds.

Remove the bowl from the mixer and use a rubber spatula to thoroughly combine ( also add and combine 4 ounces chocolate chunks ).

Pour the brownie batter into the prepared cake pan, spreading evenly. Bake the brownie for 30 minutes, until toothpick inserted in the center comes out clean. Allow to cool in the pan at room temperature for 5 minutes. Turn out onto a cake circle and refrigerate for 15 to 20 minutes. Remove the brownie from the refrigerator and cut in half horizontally. Keep the brownie at room temperature until needed.