1. Combine tomatoes and boiling water in a small bowl; let stand 10 minutes. Drain.
2. While tomatoes stand, hollow out top half of loaf, leaving a 1/2-inch border around top inside edge. Reserve bread pieces for another use.
3. Process tomatoes, cream cheese, garlic, and pepper in a food processor until smooth. Spread tomato mixture on cut sides of bread halves. Layer 10 basil leaves on bottom bread half, and top evenly with half of prosciutto. Top evenly with pepper slices and remaining prosciutto. Drizzle with balsamic vinegar. Layer remaining 10 basil leaves over sandwich; place top bread half on sandwich.
4. Wrap sandwich in plastic wrap, and place on a baking sheet; place another baking sheet on top of sandwich, and weigh down with several heavy cans. Chill 8 hours or overnight. Cut into 6 equal servings.