Place two pieces of the chicken in a gallon-sized zip-top bag. With rolling pin or the flat side of a meat mallet pound the chicken to an even 1/4-inch thickness. Repeat with the remaining chicken.
In a bowl or container, stir together the buttermilk, sriracha, 1 teaspoon of the salt salt, and the onion powder. Add the chicken, toss to coat in the marinade, cover, and refrigerate to marinade for at least 1 hour and up to overnight.
When ready to make the sandwiches, place rack in the center of your oven and preheat to 425℉. Place an oven-safe wire rack on top a rimmed baking sheet. Coat the rack with nonstick spray.
In a shallow dish, combine the panko, remaining 1 teaspoon salt, and cayenne. Working one at a time, remove a piece of chicken from the marinade, shake off the excess, and dredge it on both sides in the panko mixture, pressing as needed to coat. Set the coated chicken on the prepared wire rack. Mist the chicken with nonstick spray.
Bake the spicy chicken for 20 to 25 minutes, until the chicken reaches 165℉ on an instant-read thermometer.
While the chicken cooks, make the sauce: In a small bowl, combine the yogurt, sriracha, and salt.
To serve, toast the buns (if desired—this is easy to do in the oven) and spread sauce on the top and bottom. Add the lettuce to the bottom bun, followed by the tomato and chicken. Top with pickles.
Notes
In place of buttermilk, you can use 1 cup of whole or low-fat milk mixed with 1 tablespoon of white vinegar.
The cooking time will depend on the size of the chicken breasts, it’s best to look for smaller ones (they’re also easier to fit into a sandwich)
You could swap in chicken thighs (the size would even work better)
TO STORE: Store leftover chicken separately in an airtight container in the refrigerator for 3 to 4 days. The sauce will last in an airtight container in the refrigerator for 3 to 4 days.
TO REHEAT: Reheat spicy chicken in an air fryer, toaster oven, or at 350 degrees F in the oven so the chicken re-crisps.
TO FREEZE: The texture of thawed and re-warmed frozen spicy chicken will not be as crispy, but you can freeze the leftover chicken for up to 3 months in an airtight, freezer safe container.