Preheat oven to 400°F. Lightly butter 1 side of each bread slice.
Place 2 bread slices, buttered side down, on work surface. Top each with 2 prosciutto slices, then 4 basil leaves, then 4 cheese slices.
Sprinkle with salt and crushed red pepper. Top with remaining 2 bread slices, buttered side up.
Heat olive oil in heavy large ovenproof skillet over medium-high heat.
Add sandwiches to skillet and cook until golden on bottom, about 4 minutes. Turn sandwiches over; transfer skillet to oven and bake until golden and cheese melts, about 5 minutes.
Remove from oven. Carefully lift off top bread slices from sandwiches and insert 3 tomato slices into each, then cover with bread tops and serve.