For the soup, melt butter in a large saucepan over medium heat. Add tomato paste and cook 2 to 3 minutes, stirring frequently, until darkened in color. Stir in broth, tomato sauce, water and sugar. Bring to a boil over high heat; reduce heat to medium and simmer 15 minutes. 

While the soup simmers, place 8 slices of bread on a cutting board and brush evenly with butter on one side. Turn 4 slices over, buttered-side down, and top each with 2 slices cheese. Cover with remaining bread slices, buttered-side up. Heat a large nonstick skillet over medium heat for 1 minute. Place 2 sandwiches in the skillet and reduce heat to medium-low. Cook about 2 minutes until bottoms are golden brown. Flip sandwiches over; cook 2 to 3 minutes until cheese is melted and bread is second side is golden brown. Transfer to a wire rack and repeat with the remaining 2 sandwiches. 

Season the soup with pepper and divide among 4 bowls. Using a serrated knife, cut the sandwiches into 1-inch cubes to make croutons. Top each bowl of soup with a few croutons. Serve the remaining croutons on the side to add gradually.