Peel and slice into thin slices a medium-sized cucumber that does not contain large seeds. Place the slices in very cold water to make them crisp. Slice bread about 1/4 inch thick and spread the slices with butter. Place thin slices of cucumber on one piece, spread with thick salad dressing, and put a lettuce leaf on top of this, if desired. Cover with the second slice of bread, trim the edges, and serve.