01 - Preheat oven to 350°F or heat a large skillet over medium-low heat.
02 - Without separating the Hawaiian rolls, slice them horizontally to create a top and bottom slab.
03 - Place the bottom slab cut-side up on a cutting board. Arrange cheddar slices evenly over the rolls. Add Swiss cheese if desired. Place the top slab back over the cheese.
04 - Spread softened butter evenly on both top and bottom exteriors of the assembled rolls.
05 - For skillet: Place the entire sandwich slab in the skillet and grill for 3–4 minutes per side, pressing gently, until the bread is golden and the cheese is melted. For oven: Arrange on a baking dish and bake 10–12 minutes, until the cheese is gooey and the top is golden.
06 - Remove from heat and allow to cool briefly. Cut or pull apart into 12 mini sandwiches.
07 - Optionally, brush tops with Dijon mustard and sprinkle with chopped chives or parsley before serving.