Heat oven to 350 degrees. Combine butter and sugar in large mixer bowl. Beat at medium speed, scraping bowl often, until creamy (1 to 2 minutes).
Reduce speed to low. Add flour and cornstarch; beat, scraping bowl often, until well mixed (1 to 2 minutes). By hand, stir in chocolate chips and the nuts if desired.
Press mixture into ungreased 13x9-inch baking pan. Prick all over with fork.
Bake at 350 degrees for 35 to 45 minutes until edges just begin to brown. If browning too quickly, cover with aluminum foil.
Cool in pan 15 minutes. Cut into bars while still warm. Makes 36 bars.