Preheat oven to 325 degrees F. Whisk together flour, cornstarch, and salt in large bowl, and set aside. Beat butter, sugar, maple syrup, and vanilla extract with electric mixer 4 minutes, or until light and fluffy. Add flour mixture and mix until combined. Stir in 1/2 cup chocolate chips.

Transfer dough to 15- x 11-inch ungreased jelly-roll pan. Press dough smooth with fingers, and prick all over with fork. Bake 35 minutes, or until edges just begin to brown and dough looks slightly firm. Remove from oven and, using thin knife, cut shortbread into approximately 3- x 1-inch pieces in pan. (First cut 4 3-inch rows lengthwise; then cut 1-inch pieces crosswise.) Allow shortbread to cool completely in pan on wire rack.

Meanwhile, melt remaining 1 cup chocolate chips in top of double boiler on low heat, or melt in glass bowl in microwave 30 seconds to1 minute. Spread melted chocolate onto shortbread, spreading almost to edges. Refrigerate 15 to 20 minutes to allow chocolate topping to set. Break apart, and serve.