How did I miss a thread about toffee bars!?
A note on baking toffee bars--if you want a softer bar, don't use a cookie sheet; use a jelly roll pan. Spreading the toffee bar dough too thinly on a cookie sheet will lead to a super-crispy toffee bar, which is fine if that's what you're going for. A jelly roll pan will allow you to maintain a more uniform thickness when you spread the dough out, leading to a less crispy edge and a chewier bar.
And Xazy, my mom and I bake up a whole mess of these at Christmas every year--they're one of my favorite cookies. I think they're probably the second cookie I mastered. We also substitute a broken-up Hershey bar for the chocolate chips, and top them with chopped walnuts.
Shesus, you should not need to grease the cookie sheet or jelly roll pan; the dough for the toffee bar should contain enough butter to release the bars easily. Lining the bottom of the jelly roll pan with parchment paper is a good call, though, as the easiest way to get the bars out of the pan is to cut them up into squares after they've been covered in chocolate and remove them from the pan individually; the parchment will prevent you from scratching the pan with your knife (or else just use a dull knife
).