Butter one side of each slice of sourdough bread. Place the bread on a baking sheet lined with tin foil under the broiler for 40-50 seconds on each side.
Slice the tomatoes into thin slices.
Spread the mayonnaise on the buttered side of each slice of bread. Arrange the sliced tomato, basil, salt, and pepper on one slice of the bread, and top with the other slice.