For the marinade/batter: Use kitchen scissors to cut 6 to 7 small horizontal slivers, about 1 1/2 to 2 inches deep, into each chicken breast, to give them potential for crispies when fried. Set aside.
Whisk together the buttermilk, hot sauce, paprika, a nice pinch of salt and plenty of grinds of pepper in a medium bowl. Add to a resealable bag or large bowl along with the chicken and soak for 2 hours at room temperature or overnight in the refrigerator. Bring the chicken to room temperature before frying.
Heat several inches of oil in a pot or deep-fryer to 375 degrees F.
Remove the chicken from the marinade and pat dry (reserving the marinade), then set aside. Whisk together the flour, cornstarch and baking powder in a large bowl. Slowly add the marinade liquid while whisking to form a pancake batter consistency. (If you go too far, just add more flour or about 2 tablespoons of water, depending on the texture.) Dust the chicken with some flour, then dunk in the batter to coat. Dust the chicken again with flour, then shake gently and slowly drop into the hot oil. Fry in batches until cooked through and golden on the outside, about 8 minutes. Remove and season with a pinch of salt and rest briefly on a few scrunched paper towels on a plate.
For the sandwich: Butter both sides of each bun. Toast lightly in a grill pan or in the oven until lightly golden brown and warm, about 2 minutes.
Stir together the mayonnaise and chipotle in a small bowl. Smear on the cut sides of the bun bottoms, then add the chicken, pickles and the bun tops. Wrap in aluminum foil pouches and let rest for less than 1 minute, then serve.