Place 1/2 cup of the cucumber, mayonnaise, buttermilk, chives, lemon juice, mustard, celery seeds, and salt into a blender and puree until smooth; set aside.
Cook the farfalle according to directions on package, leaving out added fat or salt. Drain, rinse under cold water, and drain again. Place in a large serving bowl.
In the meantime, spray a large nonstick pan with nonstick cooking spray and place over medium high heat. Add in the chicken and thoroughly cook, about 4 minutes per side. Remove from pan and place on a cutting board. Cool for 5 minutes then thinly slice on an angle. Add into the bowl with the pasta.
Place the watercress, tomatoes, scallions, and remaining 1/2 cup of cucumbers into the bowl and add the dressing from the blender. Toss well to coat and serve immediately.