Grate the cheddar cheese using a box grater or food processor.
In a medium bowl, combine the mayonnaise, garlic powder, and kosher salt.
Add the whipped cream cheese and pimentos to the mayonnaise mixture and mix well.
Add the grated cheddar cheese and stir until fully combined.
Heat a skillet or griddle over medium heat.
Butter one side of each of the 4 bread slices.
Place 2 slices buttered-side down. Spread ⅓ cup of pimento cheese evenly over each. Top with the remaining 2 slices, buttered-side up.
Place the sandwiches on the heated skillet or griddle and cover with a lid for about 1 minute. Uncover and cook for 2 more minutes, until golden brown. Flip, cover for 1 minute, then uncover and cook for about 2 more minutes, until the second side is golden and the cheese is melted.