Place chicken on a cutting board. Pound lightly or slice horizontally so each piece is thin and even.
Season chicken on both sides with salt, paprika, chili powder, garlic powder, onion powder, black pepper, and cayenne pepper.
In a medium bowl, whisk buttermilk and egg.
In another bowl, mix flour, cornstarch, and baking powder.
Add chicken to buttermilk mixture. Let sit 10 minutes.
In a small bowl, mix mayonnaise, chipotle peppers, adobo sauce, lemon juice, and garlic powder. Refrigerate until ready to use.
Cook bacon in a skillet until crisp. Drain on paper towels.
Heat about 1/2-inch oil in a large skillet to 350°F
Remove chicken from buttermilk, letting excess drip off. Dredge well in flour mixture. Press coating onto both sides.
Fry chicken in batches until deep golden brown and cooked through, about 4 - 5 minutes per side. Drain on a wire rack or paper towels.
Toast sourdough bread with butter in a skillet or on a griddle until golden.
Place 1 slice American cheese on each hot piece of chicken.
Spread chipotle sauce on one side of each toasted bread slice.
Build each sandwich with sourdough, chicken, bacon, lettuce, tomato, and top bread.
Serve hot. Serve with fries, curly fries, or extra chipotle sauce on the side.