01 - Preheat oven to 350°F or heat a large skillet over medium-low heat.
02 - With rolls intact, slice horizontally with a bread knife to create a top and bottom slab.
03 - Place the bottom slab on a cutting board. Arrange cheddar and, if desired, Swiss cheese evenly over the rolls. Position the top slab over the cheese.
04 - Spread softened butter uniformly on the exterior of both the top and bottom slabs.
05 - For skillet: Place assembled rolls in skillet and grill 3 to 4 minutes per side, pressing gently, until golden and the cheese is melted. For oven: Set rolls in a baking dish and bake for 10 to 12 minutes, until the bread is golden and cheese is gooey.
06 - Let cool briefly. Cut or pull apart into 12 mini sandwiches.
07 - If using, brush tops with Dijon mustard and sprinkle with chives or parsley before serving.