Preheat oven to 350°
Line a 15 x 10 inch jelly roll pan or a 12 x 17 inch cookie sheet with foil and spray with non-stick cooking oil.
Lay a flat layer of saltine crackers out on the foil (I usually cut the final row of crackers with a pair of scissors so that they fit exactly within the edges of the pan).
In a medium saucepan, melt the butter and the brown sugar until they reach a boil.
Reduce the heat and bring to a simmer for 5-6 minutes or until the mixture is thickened and the sugar is completely dissolved. It should be pretty thick, but thin enough to pour and spread on its own.
Pour this mixture over the crackers, and spread evenly with a spatula, ensuring that all crackers are evenly coated.
Bake for 8-10 minutes or until the toffee becomes bubbly.
After removing the pan from the oven, let it sit for 3-5 minutes to let it set up. Sprinkle on the chocolate (or other variety) chips and allow them to soften and melt for about 5 minutes. Then spread them into an even layer.
You can add nuts, crushed pretzel pieces, cracker crumbs or whatever else floats your boat at this point.
Let cool and break into bite-size pieces. To expedite the process, you can refrigerate the pan for a few minutes.