Cucumber Tea Sandwiches with Tarragon Butter

1 large English cucumber, peeled, sliced paper thin
1/2 teaspoon salt
2 tablespoons white vinegar
1 cup unsalted butter, softened
1/4 cup minced fresh tarragon
1/4 cup minced fresh chervil
30 thin slices whole-wheat bread, enough to make 72 (2x4inch) rectangles
Watercress leaves (optional)
Mayonnaise and chopped fresh parsley (optional)

Put cucumber slices into a large, non-metal bowl. Toss with salt. Sprinkle with vinegar. Toss to mix well. Let stand 1 hour. Drain well in colander. Blend 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. Optional: For looks, dip one end of a sandwich into mayonnaise, then into minced fresh parsley.