The sandwiches can be assembled an hour or two before grilling (keep them stacked between pieces of waxed paper). From Napa Style catalog. The contrast of crisp, hot, cool, and creamy in this sandwich makes it a big hit.
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.
Cook in panini grill until nicely browned (2-3 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.