Heat a large pot with 1-2 quarts of oil to 375 degrees F. Slice the calamari tubes into 1/2-inch rings, and trim the tentacles if needed.
Mix the cornstarch, flour, cornmeal, salt, garlic powder, cayenne, and pepper in a pan. Pat the calamari rings dry, then dredge them in the flour mixture.
Shake off the excess flour, then working in batches gently place the calamari down in the oil. Stir so the calamari will separate. Cook for 30 seconds to 2 minutes, until the breading in light golden brown. Remove with a skimmer and place on a paper towel lined plate. Repeat with the rest. Keep warm in a 200 degree oven if not eating immediately.
For the Aioli:
Place the egg yolks, garlic, and cayenne in a food processor. Puree until lighter in color and texture, 1-2 minutes. Keep the food processor running, and very slowly drizzle in the oil until the aioli thickens. Blend in the lemon zest, and half the juice. Taste, then season with salt, pepper, and more lemon juice if needed. Set aside.
For the Salad:
Tear the watercress off the stems and the basil leaves into large pieces. Arrange the greens on plates. Top with halved cherry tomatoes, red onions, and caperberries. Place a portion of calamari on the top of each salad and serve with aioli.