Preheat oven to 350 degrees. In a medium bowl, combine flour, sugar and cocoa; cut in butter until crumbly - mixture will be dry. Press firmly on bottom of 9x13"pan. Bake 15 minutes. Meanwhile in a medium saucepan, over medium heat, melt 1 cup chips with sweetened condensed milk and vanilla. Pour evenly over baked cuts. Top with nuts and remaining 1 cup chips; press down firmly. Bake 20 minutes longer or until set. Cool.