Light Brown Sugar vs. Dark Brown Sugar. You are going to get a deeper flavor by using dark brown sugar. It has more molasses in it but if you don't have it or can't find it you can definitely use light brown sugar.
Don't Overbake. With the darker color of the dough it can happen easily. You want the cookies set around the edges but still puffy in the middle, may even look a little raw between the cracks.
Don't skip the egg yolk! This helps make the center fudgy and rich.