01 - Preheat the oven to 350°F or heat a large skillet over medium-low heat.
02 - Keeping the rolls attached, slice them horizontally to create a top and bottom layer.
03 - Place the bottom half of the rolls on a cutting board. Arrange cheddar cheese and Swiss cheese, if using, evenly over the bread. Cover with the top half of the rolls.
04 - Evenly spread softened butter on the exterior of both the top and bottom of the assembled rolls.
05 - For skillet: Grill the entire block in the pan for 3 to 4 minutes per side, pressing gently, until bread is golden and cheese is melted. For oven: Place buttered rolls in a baking dish and bake for 10 to 12 minutes until cheese is gooey and tops are golden.
06 - Cool slightly, then cut or pull apart into 12 mini sandwiches.
07 - Optionally, brush tops with Dijon mustard and garnish with chives or parsley before serving.