In a large bowl, add your pimento cheese ingredients: shredded cheese, mayonnaise, cream cheese, pimentos, onion, hot sauce, garlic powder, salt, and pepper. Using a hand mixer, stand mixer, or a wooden spoon, beat until evenly combined. Taste and add more hot sauce, salt, and pepper as desired.
Heat a skillet or griddle over medium heat. Spread butter onto one side of each bread slice. Place a bread slice, buttered side down, into skillet. Top with ½ cup of pimento cheese, spreading into an even layer. Top with bacon and another bread slice, buttered side up. Repeat for each additional sandwich.
Cook until golden brown on the bottom, about 2-3 minutes. Flip, and cook another 2-3 minutes on the second side, or until golden brown and melty.
Prepared pimento cheese will keep for up to 2 weeks in a sealed container in the refrigerator, and can easily be made ahead of time.
Serve grilled cheese paired with a side salad or soup for a complete meal.