Allow cream cheese and eggs to stand at room temperature for 30 minutes. Meanwhile, for crust, place 2 cups of the coarsely chopped Fudgy Brownies in a food processor. Cover and process with on/off pulses to form crumbs. Add melted butter; cover and process just until combined. Press mixture onto the bottom and 1 3/4 inches up the sides of a 9-inch springform pan. Chill while preparing filling. Set aside the remaining 1 cup coarsely chopped brownies.
Preheat oven to 350 degrees F. In a small heavy saucepan cook and stir bittersweet chocolate and unsweetened chocolate over low heat until melted; cool slightly.
For filling, in a large mixing bowl beat cream cheese, sugar, flour, and vanilla with an electric mixer on medium speed until smooth. With the mixer running, slowly add melted chocolate, beating on low speed just until combined. Stir in eggs and buttermilk.
Pour filling into crust-lined pan, spreading evenly. Place springform pan in a shallow baking pan. Top with the reserved 1 cup coarsely chopped brownies. Bake for 35 to 40 minutes or until a 2 1/2-inch area around outside edge appears set when gently shaken.
Cool in springform pan on a wire rack for 15 minutes. Using a small sharp knife, loosen crust from sides of pan. Cool for 30 minutes more. Remove sides of pan; cool cheesecake completely on wire rack.
In a medium saucepan cook and stir butter and chocolate over low heat until melted; cool. Preheat oven to 350 degrees F. Line an 8x8x2-inch baking pan with foil, extending the foil over edges of pan. Grease foil; set pan aside.
Stir sugar into cooled chocolate mixture. Add eggs, one at a time, beating with a wooden spoon after each addition just until combined. Stir in vanilla. In a small bowl stir together flour and baking soda. Add flour mixture to chocolate mixture; stir just until combined. If desired, stir in nuts. Pour batter into the prepared baking pan, spreading evenly.
Bake for 30 minutes. Cool in pan on a wire rack. If desired, spread Chocolate-Cream Cheese Frosting over brownies. Using the edges of the foil, lift uncut brownies out of pan. Cut into brownies.
In a small saucepan cook and stir 1 cup semisweet chocolate pieces over low heat until melted; cool. In a medium bowl stir together two 3-ounce packages softened cream cheese and 1/2 cup powdered sugar. Stir in melted chocolate until smooth.