Double the Spoonfuls

0 In a small bowl combine softened butter, grated parmesan, and garlic powder (if using), until smooth and combined.
1 Spread the butter mixture on to the outside of each of the four slices of bread, making sure to fully coat each slice including the edges.
2 Flip all four slices over, then spread a few dashes of dijon mustard on to two of the slices of bread. You can add as much as you like, but I personally find dijon can be overpowering so I don't recommend coating the slices completely.
3 Divide the shredded mozzarella between the two slices with dijon, then top each with 2-3 slices of cheddar depending on the size of your pieces of sourdough. Sprinkle with black pepper then top each sandwich with the other piece of sourdough, making sure that the buttered side is facing out.
4 In a large skillet, melt remaining tablespoon butter (or enough to just coat the bottom of the skillet, you don't want to add too much) over medium heat. Add grilled cheeses, pressing down lightly, and cook for 3-5 minutes until golden. Flip and continue cooking until cheese is melted and bottom pieces are golden. Remove from skillet, sprinkle with flaky salt, and serve immediately.