Line 12x17 cookie sheet with foil and spray well with cooking spray.
Layer the saltine crackers on top of the foil.
Heat butter and sugar on stovetop and heat until gently boiling, stirring constantly.
Reduce heat to low boiling and cook, uncovered, between 3 and 5 minutes, until the mixture is thickened and sugar is dissolved. (Be careful to avoid burning the butter and sugar- that will ruin the recipe, but do make sure the sugar is dissolved and mixture is thickening. This is by far the trickiest part of the recipe ~ just make sure it thickens and dissolves but does not burn.)
Pour butter and sugar mixture over crackers and spread evenly.
Bake 5-6 minutes in oven.
Remove pan from oven and let cool 3 minutes, then sprinkle chocolate chips evenly over brown sugar mixture.
As chocolate chips melt, carefully spread into an even layer over the toffee layer using a knife. (Note - I pop the pan back in the oven for about 20 seconds after sprinkling on chocolate chips so that they melt faster.)
Refrigerate for one hour.
Break into bite-sized pieces and serve. . . yum, store covered in refrigerator.