Mix 2 tablespoons all-purpose flour, 1 tablespoon sugar, 1 teaspoon dry mustard, 1/2 teaspoon salt, and dash cayenne; stir in 2 slightly beaten egg yolks and 3/4 cup skim milk.
Cook and stir over very low heat till thick and bubbly.
Stir in 3 tablespoons vinegar; chill.
Drain and slice one 5-ounce can water chestnuts.
Roll edges of a few water chestnuts in paprika; set aside.
Spread 8 slices whole wheat bread with some of the dressing; top each slice with 5 sprigs watercress, 1 slice cooked chicken, and a few remaining water chestnuts.
Top with a few slices paprika-edged water chestnuts.
Pass remaining dressing.