Heirloom Tomatoes with Bacon, Blue Cheese, and Basil

Cut 1 round from each bread slice with cookie cutter. Cook bacon in a 10-inch heavy skillet over moderate heat until crisp, then transfer to paper towels to drain. Pour off bacon fat from skillet and reserve fat (do not clean skillet). Heat 1 1/2 tablespoons oil in skillet over moderate heat until hot but not smoking, then toast 3 bread rounds, turning over once, until golden brown, about 3 minutes total. Transfer toasts to a rack to cool and season with salt and pepper. Toast remaining 3 bread rounds in 1 1/2 tablespoons more oil in same manner. Cook shallot in 2 tablespoons reserved bacon fat and remaining 3 tablespoons oil in a small heavy saucepan over moderate heat, stirring, until softened, about 2 minutes. Add vinegar and simmer, whisking, until emulsified, about 1 minute. Season dressing with salt and pepper and keep warm, covered. Crumble bacon. Arrange bread rounds on 6 plates and divide tomato slices among them, stacking slices and sprinkling some basil and bacon between slices. Sprinkle cheese and remaining basil and bacon over and around tomatoes. Spoon some of warm bacon dressing over and around tomatoes and season with salt and pepper.