In a large bowl, beat butter with sugar until fluffy; beat in vanilla. Whish together flour, cornstarch and salt;with a wooden spoon, blend into butter mixture, 1/4 cup at a time. Stir in chocolate.
Pat dough evenly onto parchment paper-lined 15- x 10-inch baking sheet. Refrigerate for 30 minutes or freeze for 15 minutes. Deeply score into 2- x 1-inch bars.
Topping: Sprinkle with sugar. Bake in 300 degree F oven until light golden, 50 to 55 minutes. Let cool slightly on rack. Cut into bars while still warm. Let cool.