1. In a medium saucepan, melt and stir 4 ounces (3/4 cup) of the dark chocolate pieces, butter, and the unsweetened chocolate over low heat. Remove from heat; cool.
2. Preheat oven to 350 degrees F. Line a 13x9x2-inch baking pan with foil, extending the foil over the edges of the pan. Grease foil; set aside.
3. Stir sugar into the cooled chocolate mixture in saucepan. Add the eggs, one at a time, beating with a wooden spoon just until combined. Stir in vanilla.
4. In a small bowl, stir together the flour, baking soda, and salt. Add flour mixture to chocolate mixture, stirring just until combined. Stir in the remaining 7 ounces (1-1/2 cups) dark chocolate pieces. Spread batter evenly in the prepared pan.
5. Bake in the preheated oven for 30 minutes. Cool in pan on a wire rack. Spread Creme de Menthe Filling over cooled brownies. Place uncut brownies in the refrigerator while preparing the Dark Chocolate Ganache.
6. Spread slightly cooled Dark Chocolate Ganache over Creme de Menthe Filling, spreading to the edges with a spatula. Cover and chill about 1 hour or until set. Using the edges of the foil, lift the uncut brownies out of the pan. Cut into bars. Makes 48 brownies.