In a medium bowl, combine the shredded chicken and barbecue sauce. Mix until the chicken is well coated.
Heat a skillet over medium heat. While the skillet is heating, spread butter on one side of each slice of bread.
Place four slices of bread, buttered side down, in the skillet.
Layer each slice with two slices of cheddar cheese, then evenly distribute the BBQ chicken mixture on top of the cheese. If using, sprinkle green onions over the chicken.
Top each sandwich with another two slices of cheddar cheese and the remaining slices of bread, buttered side up.
Cook for about 4-5 minutes, or until the bottom is golden brown and the cheese begins to melt. Carefully flip the sandwiches and cook for another 4-5 minutes on the other side.
Once both sides are golden and the cheese is melted, remove from the skillet and let cool for a minute before slicing in half.