Preheat oven to 425 F. Cover a 15x10x1 inch pan with aluminum foil. Line up the 40 saltines flat on the pan.
In a small saucepan, melt butter and brown sugar and bring to a boil. Boil 5 minutes, stirring occasionally. Remove from heat and pour over crackers, spreading if necessary to cover them evenly.
Bake 2 to 4 minutes. Don't walk away. You need to watch them to make sure they don't scorch. When they are just starting to bubble they are done. Take them out and pour the chocolate chips over the top. Wait 3 or 4 minutes, then spread the chocolate with a knife.
I ate them at this point. The recipe says to freeze them for 15 minutes, but I don't like cold chocolate so I just got melty goodness all over my fingers. Do what you want. Break the sheet into pieces and store in a tupperware on the countertop.