Beat 1/2 cup butter at medium speed with an electric mixer until creamy. Gradually add powdered sugar, beating well. Add vanilla; beat well. Gradually add flour, beating at low speed until blended.
With floured fingertips, press dough into a lightly greased 8" square pan. Bake at 325° for 25 minutes or until golden brown. Let cool on a wire rack.
Place granulated sugar in a medium-size heavy skillet. Cook, stirring constantly, over medium heat 8 minutes or until sugar is caramel-colored. (Sugar will become crumbly before caramelizing.) Reduce heat to low; gradually add 2/3 cup whipping cream, stirring to combine. (Mixture will sputter.) Add 1 cup butter, 4 pieces at a time, stirring until smooth after each addition (about 7 minutes). Remove caramel from heat; let cool 10 minutes.
Sprinkle nuts over baked crust. Pour caramel over nuts. Cool completely.
Place remaining 1/3 cup whipping cream in a microwave-safe glass measuring cup. Microwave at HIGH 1 minute or until hot. Pour hot cream over chopped chocolate in a bowl. Let stand 2 minutes. Whisk until smooth. Pour chocolate over cooled caramel, spreading chocolate to edges. Cover and refrigerate until layers are firm. Keep refrigerated until ready to serve. Let stand 5 minutes at room temperature before cutting and removing bars from pan.