In a medium mixing bowl using an electric mixer, beat 12 tbsp. butter with sugar until light and fluffy. Add egg yolk; mix just until fully incorporated. In a separate small mixing bowl, mix together flour and salt; add to bowl with the butter mixture. Stir until no traces of flour remain. Press mixture evenly and firmly into bottom of a 9x13" nonstick pan (sides only sprayed with cooking spray). Bake in a preheated 350? oven until light golden-brown (12-16 min.). Remove from oven; allow to cool slightly (leave the oven on). In a small saucepan over medium heat, combine sweetened condensed milk and remaining butter; cook, stirring frequently with a silicone spatula until bubbly, then 5 min. more until thick. Pour evenly over crust; bake 10 min. Remove from oven; sprinkle evenly with chocolate chips. Return to oven; bake 3-5 min. until chocolate begins to melt. Remove from oven; spread chocolate with a knife evenly over the surface of the filling. Sprinkle toffee pieces evenly over melted chocolate, pressing in lightly. Cool at room temperature 30 min. Cool completely in the refrigerator; cut into 1 1/2x 4" bars. Store tightly covered in the refrigerator up to 5 days.