Preheat oven to 400 degrees. Place crackers in foil-lined 15x10x1 inch baking pan.
Place butter and sugar in saucepan; cook on medium-high heat until butter is completely melted and mixture is well blended, stirring occasionally. Bring to a boil'; boil 3 minutes without stirring. Spread over crackers.
Bake 5 to 7 minutes or until topping is golden brown. Immediately sprinkle with the chopped chocolate; let stand 5 minutes or until chocolate is softened. Spread chocolate evenly over ingredients in pan; sprinkle with walnuts. Cool. Break into pieces.