In a mixing bowl, combine the shredded chicken and buffalo sauce. Stir until the chicken is evenly coated.
Preheat a skillet or frying pan over medium heat.
Butter one side of each slice of bread. On the unbuttered side, lay out 4 slices in the pan.
Add a layer of shredded mozzarella cheese (about 1/2 cup) on top of each slice in the pan.
Add a generous portion of the buffalo chicken mixture on top of the cheese.
If using, sprinkle blue cheese crumbles on top of the chicken.
Add another 1/2 cup of shredded mozzarella cheese over the chicken.
Top with the remaining slices of bread, buttered side facing out.
Cook for about 3-4 minutes on one side, or until the bread is golden brown and the cheese starts to melt. Carefully flip the sandwiches using a spatula and cook the other side for another 3-4 minutes until golden brown and the cheese is fully melted.
Remove from skillet, slice in half, and serve hot. Garnish with chopped green onions if desired.