If a nice, even crispiness is your priority when it comes to grilled cheese, skip the stove and try the air fryer. You'll get a uniform texture and lovely melt without having to worry about burning your sandwich or any flipping mishaps.
Preheat air fryer to 375°F for 5 minutes. Spread butter evenly on 1 side of all 4 slices of bread. Place 2 slices of bread, buttered-side down in air fryer. Top each piece with 1 slice of Cheddar and 1 slice of Havarti. Place remaining bread slices on top of each sandwich, buttered side up.
Cook 7 minutes. Flip, and cook an additional 3 minutes, or until golden and toasted all around.