Step 1 can be completed 1 hour in advance, and the cucumber slices stored in the refrigerator.
1. Using a sharp knife, carefully slice the cucumber into rounds, no thicker than 1/8 in (2mm). Place the cucumber slices in a colander and sprinkle with the salt. Let stand for 20-30 minutes to allow the excess liquid to drain off. When ready to use, sprinkle the slices with the vinegar.
2. Spread each slice of bread with some softened butter. Arrange the cucumber slices in an even layer over 4 slices of bread. Season with pepper, then top with the remaining bread slices, buttered sides down.
3. Cut off the crusts from each sandwich, then cut each sandwich into squares, triangles, or rectangles. Arrange the sandwiches on a serving plate. Sprinkle with watercress, and serve at once.
Cheese and Cucumber Sandwiches
As an alternative to butter, spread the bread with cream cheese or soft goat cheese.
Nutritional information includes 4 teaspoons of butter for spreading and using white bread.