Preheat oven to 350 degrees, using the middle rack.
Line a 10-inch by 15-inch jelly roll pan with foil. (You can also use a cookie sheet, but be sure to turn the edges of the foil up) Spray the foil with cooking spray.
Line the pan completely with crackers, salt side up. Cover the whole bottom. Set aside.
Combine the butter and brown sugar in a saucepan. Bring to a boil over medium high heat, stirring constantly. Boil for exactly 5 minutes, stirring constantly. If it sputters, reduce the heat. If it stops boiling, reduce the heat. NEVER STOP STIRRING.
Pour the mixture over the crackers as evenly as possible.
Slide the pan into the oven. Bake at 350 degrees for 10 minutes.
Remove pan and sprinkle with chocolate chips. After a minute or two of melting, spread the chips with a spatula.
Sprinkle chopped pecans over the top and refrigerate.
When the toffee has chilled thoroughly, peel it from the foil and break it into random sized pieces.