Coat a 13" x 9" pan with cooking spray. Line pan with aluminum foil, allowing ends to hang over short sides of pan. Tuck overlapping ends under rim on short sides. Coat foil with cooking spray; set pan aside.
Combine flour and next 3 ingredients in a small bowl. Place chopped semisweet chocolate in a large bowl; set aside.
Combine sugar and next 3 ingredients in a saucepan; cook over medium heat, stirring constantly, until sugar and butter melt and mixture comes to a rolling boil. Remove from heat, and pour over chopped chocolate in bowl; let stand 2 minutes (do not stir).
Beat mixture at low speed of an electric mixer until chocolate melts and mixture is smooth. Add eggs, one at a time, beating well after each addition. Add flour mixture; beat at medium speed until well blended. Stir in vanilla, walnuts, and dark chocolate.
Spoon batter into prepared pan, spreading evenly. Bake at 325° for 45 to 48 minutes. Cool completely in pan on a wire rack. Cover brownies with overlapping foil; chill at least 2 hours.
Carefully invert brownies from pan, using overlapping foil as handles; remove foil. Invert brownies again onto a cutting board; cut into squares or diamonds.