The Best Blts

Mix basil, mayonnaise and butter in processor until basil is finely chopped and mixture is well blended. Season to taste with salt and pepper. (Can be made 1 day ahead. Cover and refrigerate.)Cook bacon in heavy large skillet over medium-high heat until crisp, about 8 minutes. Transfer to paper towels; drain. Spread half of mayonnaise mixture over 1 side of 6 bread slices. Top each with 2 tomato slices. Sprinkle tomatoes with salt and pepper. Top tomato slices with avocado, then with bacon strips, onion and lettuce. Spread remaining mayonnaise mixture over remaining 6 bread slices. Place bread slices atop lettuce. Cut sandwiches in half and serve.