Pound chicken breasts to an even thickness. In a bowl, whisk buttermilk, lime juice, salt, and pepper. Submerge chicken, cover, and marinate in the fridge for at least 2 hours.

Prepare the sauce by mixing mayonnaise, sour cream, chopped cilantro, lime juice, and minced garlic in a bowl. Refrigerate until ready to use.

In a shallow dish, combine flour, cornstarch, garlic powder, onion powder, paprika, and salt. Drizzle 2 tablespoons of the buttermilk marinade into the flour and mix with a fork to create crispy crumbles.

Remove chicken from marinade, letting excess drip off. Press firmly into the flour mixture to coat completely. Let breaded chicken rest on a wire rack for 10 minutes.

Heat 1 inch of oil in a heavy skillet to 350°F (175°C). Fry chicken for 4-5 minutes per side until golden brown and cooked to an internal temperature of 165°F (74°C). Drain on a clean wire rack.

Spread cilantro lime sauce on toasted buns. Layer the bottom bun with chicken, shredded cabbage, cucumber slices, and jalapeños. Drizzle with extra sauce, top with the bun, and serve.