1. Preheat oven to 350 degree F and butter and flour a 13-x9-inch nonstick metal baking pan.
2. Melt chocolate and butter in a large heat-proof bowl set over a saucepan of simmering water, stirring until smooth. Remove bowl from pan and stir in sugars and vanilla. Add eggs 1 at a time, stirring vigorously after each addition, until combined well. Add flour and salt and stir until just combined. Stir in chopped nuts and spoon mixture into prepared pan, smoothing top.
3. Bake in middle of oven until a toothpick inserted in center comes out clean, about 40 minutes. Let brownies cool in pan on a rack.Make Peanut Butter Layer::
4. Whisk together cream cheese and peanut butter until combined well. Sift confectioners' sugar over mixture and whisk until combined well and smooth. Spread peanut butter mixture evenly over brownie layer with a spatula and chill until firm.Make Chocolate Layer::
5. Melt chocolate with butter and cream in a heat-proof bowl set over a saucepan of simmering water, stirring until smooth, about 4 minutes. Remove bowl from pan and let chocolate mixture cool to room temperature. Spread cooled mixture over peanut butter layer and chill until firm.
6. Trim brownies along one short side with a hot dry knife (reserve trimmings for a snack) and cut remaining brownies into bite-size pieces (about 1 x 1-1/2 inches). The first piece will be the hardest to get out. Keep brownies chilled, loosely covered, until ready to serve.