In a large bowl, mix together cheese, mayonnaise, bell pepper, Dijon mustard, apple cider vinegar, sriracha chili sauce, garlic. powder, onion powder, salt, pepper and cayenne pepper until well combined. Refrigerate 2 hours to thoroughly combine flavors. (Makes about 2 cups. Pimiento cheese can be stored in refrigerator up to one week.)
Bring pimento cheese to room temperature.
Heat a large nonstick skillet over medium-low heat. Spread butter on 1 side of each slice of bread. Flip 4 slices over and spread the pimiento cheese on each of slice. Top each with another slice of bread and place into preheated skillet. Cook 3-4 minutes until golden brown. Turn sandwiches and cook 3 to 4 minutes until second side is crispy and golden brown and cheese has melted. Remove from heat and serve.