In a medium saucepan heat and stir butter and unsweetened chocolate over low heat until smooth; set aside to cool. Preheat oven to 350°F. Line an 8x8x2-inch baking pan with foil, leaving about 1 inch of the foil extending over the ends of pan. Grease foil; set pan aside.
Stir the sugar into the cooled chocolate mixture. Add the eggs, one at a time, beating with a wooden spoon just until combined. Stir in vanilla. In a small bowl stir together the flour and baking soda. Add flour mixture to chocolate mixture; stir just until combined. If desired, stir in nuts. Spread the batter evenly in the prepared pan.
Bake for 30 minutes. Cool in pan on a wire rack. If desired, spread Chocolate-Cream Cheese Frosting over cooled brownies. Remove brownies from pan, using the overlapping foil to lift brownies. Place on cutting board; cut into brownies.
In a small saucepan heat and stir chocolate pieces over low heat until smooth. Remove from heat; let cool. In a medium bowl stir together cream cheese and powdered sugar. Stir in melted chocolate until smooth.