Method
Put cucumber slices in large bowl. Toss with salt. Sprinkle with vinegar. Toss to mix well. Let stand 1 hour. Drain well in colander.
Combine butter, tarragon and chervil.
To assemble, spread butter over 1 side of each bread slice. Cover 15 slices with cucumbers, dividing evenly. Close sandwiches. Trim crusts. Cut into 36 rectangles. Arrange on platter, garnished with watercress leaves.