Pulse the cheese, mayo, pimentos and cayenne in the food processor until just combined. Season with salt and pepper. Cover and chill for at least 3 hours.
Spread some additional mayo on 1 side of each slice of toasted bread. Arrange 6 slices, mayo-side down, on a clean work surface. Place 2 heaping tablespoons pimento cheese evenly on the 6 slices of bread and top with the remaining 6 slices of bread, mayo-side facing up.
In a large nonstick skillet over moderate heat, cook 3 sandwiches until the bread is golden brown and the cheese is starting to melt, about 3 minutes. Turn the sandwiches over and cook until golden brown, about 3 minutes more. Wipe the pan clean and repeat with the remaining 3 sandwiches.
What To Toss In: If you like your pimento really spicy, you can always add some chipotle in adobe sauce to give it a smoky flavor. This can also be used as an entertaining dip. Just arrange some baby carrots and cut up some celery sticks around the cheese. People will flock to it.