Lay all four bread slices on a flat surface. Spread softened butter evenly on one side of each slice.
Flip the slices over. Spread 2 tablespoons of pizza sauce onto the unbuttered side of two of the bread slices.
Layer 1½ cups of mozzarella and ½ cup of provolone evenly over the sauced bread slices.
Place pepperoni slices over the cheese. Sprinkle Italian seasoning, garlic powder, and red pepper flakes on top.
Close the sandwiches with the remaining two bread slices, buttered side facing up.
Heat a large skillet over medium to medium-low heat.
Transfer sandwiches to the pan. Cook for 3 to 4 minutes on the first side until deep golden brown.
Flip carefully and cook for 3 to 4 minutes on the second side until golden and cheese is fully melted.
Remove from heat and rest for 1 to 2 minutes before cutting.
Slice diagonally and serve immediately.