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

Lay out 6 slices of bread and spread each with a heaping tablespoon of mayonnaise mixture. Layer each piece of bread with about 9 slices of cucumber. Divide watercress between 6 slices and top with remaining 6 slices of bread.

Press down lightly to attach and trim the crusts off all 4 edges. Cut in half to form 2 small rectangular finger sandwiches.

Can be made up to 2 hours in advance if wrapped tightly in plastic wrap and refrigerated.