Preheat oven to 400 degrees F.
Line a medium-sized (close to 10 1/2" x 15 1/2" x 1") jelly roll pan with foil.
Place one layer of saltine crackers edge to edge to cover the pan.
In a heavy saucepan, heat the butter, margarine, and sugar to a boil. Boil for 3 minutes, no stirring necessary. If using all butter (2 sticks), keep a close eye on it to prevent it from burning. It may only take 2 minutes. If the butter mixture begins to turn brown, remove from heat and proceed with recipe.
Pour mixture over saltines and spread with a rubber spatula if necessary to distribute evenly over the crackers.
Bake about 6-10 minutes or until crackers are golden brown (the browner the crackers, the more toffee-tasting the candy will be).
Remove from oven and pour the chocolate chips over the crackers. Wait 1-2 minutes, then spread the chocolate to cover the entire surface. Optional: decorate with toppings of choice.
Refrigerate pan for 2 hours or longer. Break candy into pieces. Store in refrigerator for best results.