In a bowl, mix the shredded chicken with BBQ sauce until fully coated. Set aside.
Butter one side of each slice of bread. These will be the outsides of the sandwich for grilling.
Place two slices of bread, butter-side down, on a clean surface. Top each with a mix of cheeses, then add the BBQ chicken mixture. Add any optional toppings. Finish with more cheese and the second slice of bread, butter-side up.
Heat a skillet over medium heat. Add the sandwiches and cook 3–4 minutes per side, pressing lightly with a spatula, until the bread is golden brown and crispy and the cheese is melted.
Remove from the pan, let cool slightly, then slice in half and serve warm.