1. While it is a great idea to premix your dry ingredients and keep them in labeled bags, it is NOT a good idea to mix brown sugar with cookies. I needed to mix crushed Nillas with brown sugar and butter for the bottom of toffee bars. When I went to get the mix, the brown sugar had turned hard as a rock overnight. At least I hadn't yet blended it together so it was pretty easy to remove all the sugary boulders.
2. Just because a pan seems to be 9x13 doesn't mean it is. It is a good idea to measure your pans ahead of time. I put my cookie/brown sugar/butter crust in a pan I thought was the right size and it was too big. Probably 11 by 18? So I had these big gaps and not enough 'base' to hold the toffee layer. Therefore, my toffee did not break into nice chunks. In fact, it is very crumbly.
3. Crumbly toffee tastes really good.