01 - Preheat oven to 350°F or heat a large skillet over medium-low heat.
02 - Keeping the Hawaiian rolls intact, slice them horizontally with a bread knife to create a top and bottom layer.
03 - Place the bottom slab of rolls on a cutting board. Arrange cheddar slices, and Swiss if desired, evenly over the rolls. Place the top slab of rolls above the cheese.
04 - Spread softened butter evenly on the top and bottom exterior of the entire roll assembly.
05 - If using a skillet, add the roll slab and grill for 3 to 4 minutes per side, pressing gently, until the bread is golden and the cheese melted. If using the oven, transfer to a baking dish and bake for 10 to 12 minutes until cheese is gooey and tops are golden.
06 - Let cool slightly, then separate into 12 mini sandwiches.
07 - Brush tops with Dijon mustard and sprinkle chopped chives or parsley, if using, before serving.