Preheat oven to 350 degrees F (175 degrees C). Line a jelly roll pan with aluminum foil; lightly spray with cooking spray.
Arrange crackers in the prepared jelly roll pan.
Mix sugar and butter together in a saucepan; bring to a boil, stirring constantly, until sugar is dissolved, about 1 minute. Pour butter mixture over crackers.
Bake crackers in the preheated oven until lightly browned, 5 to 6 minutes. Remove from oven.
Sprinkle chocolate chips over the hot crackers; spread chocolate chips with a rubber spatula until melted. Top with walnuts. Refrigerate chocolate bark until set; break into pieces.