In a medium bowl, combine the shredded chicken with buffalo sauce, stirring well until each piece is thoroughly coated.

Take one slice of bread and layer half of the cheddar cheese evenly over it. Spoon the buffalo chicken mixture on top, then add the mozzarella cheese for that creamy stretch. Finish with the remaining cheddar cheese before placing the second slice of bread on top.

Place a skillet over medium heat and add 1 tablespoon of butter, allowing it to melt. Once melted, place the assembled sandwich in the skillet and cook for about 3-4 minutes until the bottom is golden brown.

Using a spatula, gently flip the sandwich, adding another tablespoon of butter to the skillet. Cook the other side for an additional 3-4 minutes until golden brown.

Once both sides are golden, remove the sandwich from the skillet and allow it to cool for a minute before slicing in half. Garnish with green onions or blue cheese crumbles if desired.