PREHEAT oven to 325° F.
BEAT sugar and butter in a large mixer bowl until creamy. Add egg yolk; beat well. Beat in flour. Press evenly onto bottom of ungreased 15 x 10-inch jelly-roll pan.
BAKE for 22 to 25 minutes or until lightly browned and center is still slightly soft. Sprinkle with chocolate chips (*NESTLÉ® TOLL HOUSE® Semi-Sweet Chocolate Mosels can be used instead). Let stand for 3 minutes or until morsels are shiny; spread evenly. Sprinkle with toffee chips (**you can also use 1 crushed English toffee candy bar). Cool for 1 hour; cut into bars with sharp knife while slightly warm.
Makes 4 dozen.