Preheat oven to 375 degrees. Line rimmed cookie sheet with foil and butter top of foil well. Place graham crackers side by side, covering the bottom of the cookie sheet.
Boil (for about 4 minutes) butter and brown sugar. Pour over the crackers, spreading evenly. Place in oven for 4-5 minutes or until COVERED with bubbles.
Remove from the oven and immediately sprinkle chocolate chips over the top. After they have melted, spread chopped nuts over the top. Press the nuts down lightly with your hand or they tend to fall off when stored. Refrigerate until cool. Peel off the foil and cut into squares. Store in refrigerator.