In a large bowl, combine buttermilk, garlic powder, onion powder, paprika, salt, and black pepper. Add the chicken breasts, ensuring they are fully submerged. Cover and refrigerate for at least 1 hour or up to overnight for best flavor.
Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
In a shallow dish, combine flour and panko breadcrumbs. Remove chicken from the marinade, allowing excess to drip off, then dredge each piece in the flour mixture, pressing lightly to adhere.
Place the coated chicken on the prepared baking sheet. Bake for 20-25 minutes, or until the chicken is cooked through and golden brown, reaching an internal temperature of 165°F (74°C).
While the chicken is baking, mix together mayonnaise, sweet chili sauce, and sriracha in a small bowl. Adjust the sriracha to your preferred spice level.
Toast the sandwich buns in the oven for the last 5 minutes of baking.
Assemble the sandwiches by spreading the spicy mayo on the bottom half of each bun. Add shredded lettuce, a piece of baked chicken, sliced tomato, and pickles if desired. Top with the other half of the bun.