In a large bowl, beat 1-1/2 cups butter with an electric mixer on medium to high speed for 30 seconds. Add sugar and 1-1/2 teaspoons vanilla; beat until combined, scraping sides of bowl occasionally. Beat in as much of the flour as you can with the mixer; stir in any remaining flour. Cover and chill for 30 to 60 minutes or until dough is easy to handle.

Preheat oven to 350 degrees F. Line a 13x9x2-inch baking pan with foil, extending foil over edges of pan. Press dough evenly into bottom of prepared baking pan. Bake for 20 to 25 minutes or until top is lightly browned. Cool in pan on a wire rack.

Spread dulce de leche in an even layer atop cooled crust. In a medium saucepan, heat whipping cream, 6 tablespoons butter, and the corn syrup to boiling over medium heat, stirring to dissolve syrup. Remove from heat. Add chocolate and vanilla to saucepan. Do not stir. Let stand 5 minutes. Stir until smooth. Let stand 10 minutes at room temperature to cool slightly. Slowly pour chocolate mixture over dulce de leche layer, spreading evenly. Cover and chill 1 to 2 hours or until chocolate layer is set.

Layer bars between sheets of waxed paper in an airtight container; cover. Store in the refrigerator for up to 3 days or in the freezer for up to 3 months. Use the foil to lift uncut bars out of pan. Cut into bars. Makes 48 bars.

If desired, cut bars into smaller and/or larger squares and place on serving platter.

Layer bars between sheets of waxed paper in an airtight container; cover. Store in the refrigerator for up to 3 days or in the freezer for up to 3 months.