Unroll the pancetta slices and cut into 4-inch lengths. Put the pancetta in a skillet and cook over moderate heat until it renders much of its fat and begins to crisp, about 10 minutes. Drain in a sieve.

In a small bowl, stir the basil into the mayonnaise.

Butter one side of each bread slice, then put 4 slices, buttered side down, on a work surface. Top the bread slices with the pancetta, distributing it evenly. Top each with a second slice of bread, buttered side up.

Heat a cast-iron skillet or grill pan until hot. Place the sandwiches in the skillet and top with a weight, such as another skillet. Cook until nicely browned, 2 to 3 minutes, then turn the sandwiches and replace the weight. Cook until the second side is well browned, about 2 minutes.

Transfer the sandwiches to a work surface and remove the top slice of bread from each sandwich. Spread the underside of those slices with mayonnaise. Top the pancetta with the tomato slices, pepper to taste, and romaine leaves. Replace the top slice of bread, cut the sandwiches in half, and serve immediately.