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.
Recipe courtesy of Claire Robinson, 2010