Stir chicken, hot sauce, melted butter, cream cheese, ranch, garlic powder, and green onions until creamy.
Warm a large skillet or griddle over medium-low heat.
Spread the softened butter on one side of each bread slice.
Layer cheddar on the unbuttered side of half the slices. Spoon on the Buffalo chicken, then add Monterey Jack and a pinch of blue cheese if using.
Top with remaining bread, buttered side facing out. Cook until golden, 3–4 minutes per side, lowering heat if browning too fast.
Rest 1 minute, then slice and serve with extra hot sauce or ranch for dipping.