1. Prepare your ingredients: Start by thinly slicing your fresh mozzarella and summer tomato. Aim for slices that are not too thick, so they melt and cook evenly within the sandwich. Lay out your sourdough bread slices.

2. Butter the bread: Generously spread softened butter on one side of each of the four sourdough bread slices. Ensure the butter covers the entire surface for even browning and crispiness.

3. Assemble the first sandwich: Place two slices of bread, butter-side down, in a cold or barely warm skillet. Immediately layer half of the mozzarella slices on one piece of bread, then arrange half of the tomato slices over the mozzarella. Season lightly with a pinch of salt and pepper.

4. Add the top bread: Place the second slice of bread, butter-side up, on top of the tomato and cheese layers. Repeat the assembly process for the second sandwich if your skillet is large enough to cook both at once.

5. Begin toasting: Turn the heat to medium-low. This lower heat is crucial for allowing the cheese to melt thoroughly and the bread to toast to a beautiful golden brown without burning. Cook for 4-6 minutes on the first side.

6. Flip and finish: Carefully lift one sandwich with a spatula to check for golden brown perfection. Once achieved, gently flip the sandwich. Continue cooking for another 4-6 minutes on the second side, until the bread is deeply golden and the mozzarella is completely melted and gooey.

7. Serve immediately: Once both sides are perfectly toasted and the cheese is melted, remove the sandwiches from the skillet. For the best experience, slice them in half diagonally and serve hot.