01 - Preheat oven to 350°F or heat a large skillet over medium-low heat.
02 - Keeping the Hawaiian rolls attached, slice horizontally using a bread knife to create top and bottom slabs.
03 - Place the bottom slab of rolls on a cutting board. Evenly arrange halved cheddar slices, and Swiss slices if desired, across the surface. Cover with the top slab.
04 - Spread softened unsalted butter evenly on the top and bottom exterior of the assembled rolls.
05 - For skillet method, place the whole roll slab in the pan. Grill for 3 to 4 minutes per side, pressing gently, until the bread is golden and cheese is melted. For oven method, position in a baking dish and bake for 10 to 12 minutes, until cheese is gooey and the bread is golden.
06 - Transfer rolls to a cutting board, let sit briefly, then separate into 12 mini sandwiches.
07 - If desired, brush tops with Dijon mustard and sprinkle with chopped chives or parsley before serving.