Preheat a panini maker according to the manufacturer's instructions, or heat a skillet over medium heat and lightly butter one side of each slice of bread.
Lay out 4 slices of bread, buttered side down if using a skillet. Divide the 2 cups of cooked chicken fajitas evenly among the 4 slices.
Top the chicken on each slice with one slice of Havarti cheese, then place the remaining bread slices on top to form sandwiches.
Grill the sandwiches in the panini maker for 3–4 minutes, or cook in the skillet 2–3 minutes per side, pressing gently, until the bread is golden and the cheese is melted.
Remove sandwiches, slice if desired, and serve with salsa or sour cream on the side if using.
