Preheat a large skillet or griddle over medium-low heat.
Spread 1 tablespoon of softened butter evenly on one side of each slice of sourdough bread.
Place two slices of bread, buttered side down, on the preheated skillet. Layer the mozzarella, provolone, sun-dried tomatoes, basil, garlic powder, and Italian seasoning evenly over the bread.
Top with the remaining two slices of bread, buttered side up.
Cook for 3-4 minutes on each side, or until golden brown and cheese is melted, pressing down gently with a spatula halfway through cooking.
Remove sandwiches from skillet, let cool for 1 minute, then cut in half diagonally and serve warm.