In a small bowl, combine mayonnaise, mint, and salt and stir to combine.

Lay out six slices of bread and spread each with a heaping tablespoon of mayonnaise mixture. Layer each piece of bread with about nine slices of cucumber. Divide watercress between six slices and top with remaining six slices of bread.

Press down lightly to attach and trim the crusts off all four edges. Cut in half to form two small rectangular finger sandwiches.

Can be made up to two hours in advance if wrapped tightly in plastic wrap and refrigerated.