Place the slices of bread on a flat surface. Add butter to one side of each of the slices of bread.
Add the slices of cheese to the non-buttered side of the bread. Add the grated mozzarella cheese. Place the second non-buttered side of the bread on top.
Lighty grease a non-stick pan and place it over medium heat. Once hot, add the sandwich onto the pan and let it sizzle, and cook for 2-3 minutes. Flip and cook for a further 2 minutes. Press down using a spatula and if cheese begins to ooze out, the grilled cheese is done.
Remove the grilled cheese sandwich off the pan and sprinkle with the flaky sea salt. Slice in half and serve.