In a small bowl, mix together softened butter, parmesan, garlic powder, and salt (if using unsalted butter) until combined.
Preheat a griddle or large skillet over medium-low heat.
Spread butter mixture onto one side of all four slices of bread. Top the non-buttered sides of two of the bread slices with cheese. Press remaining bread slices, butter-side up, on top.
Transfer sandwiches to griddle or skillet and cook until golden on bottom, about 5 minutes. Flip and cook until sandwiches are golden on second side and cheese is melted.
Slice and serve.