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