1. Place 2 bread slices on a work surface. Evenly spread ½ Tbsp. butter over each slice. Turn 1 slice over and spread 2 tsp. Sriracha over, then top with ½ cup Castello Aged Havarti Cheese and half the kimchi. Close sandwich with buttered sides facing out.
2. Heat ½ Tbsp. butter in a medium nonstick skillet over medium-low until foaming. Cook sandwich, gently pressing occasionally with a heatproof spatula, until underside is golden brown and crisp, about 3 minutes. Turn sandwich over and cook until golden brown on second side and cheese inside is fully melted, about 3 minutes. Transfer sandwich to a plate. Repeat process with remaining ingredients to make 1 more sandwich.