1) Pound each chicken breast 1/4 inch thick between two pieces of plastic wrap. No need to pound it hard. If you don’t have a mallet, you can use a rolling pin or bottle of wine.
2) Add breasts to buttermilk marinade to sit for at least an hour, but overnight works best.
3) Remove breasts from buttermilk and dredge in flour mixture. Let sit on a plate to let the flour stick to the chicken well.
4) Fry breasts in oil at 350 degrees for about 4 minutes per side. They should be golden brown.
5) Let chicken drain on wire racks over some paper towels or on a paper bag for a few minutes.
6) To make mayo, pulse mayo and chipotle peppers a few times in a food processor.
7) To make sandwich, toast breast well, then slather with mayo. Add chicken breast and a big handful of chopped lettuce.