Start by marinating the chicken. Place the chicken breasts in a bowl and pour buttermilk mixed with eggs over them. Cover and refrigerate for at least 1 hour or overnight for better flavor.
Combine the flour, paprika, garlic powder, onion powder, celery salt, thyme, basil, oregano, ground mustard, ground ginger, salt, and black pepper in a shallow dish. Mix well.
Heat vegetable oil in a deep skillet or deep fryer to about 350°F.
Remove the chicken breasts from the egg-buttermilk mixture, allowing any excess to drip off. Dip each chicken breast into the flour mixture, coating it evenly on both sides.
Carefully place the coated chicken breasts into the hot oil and fry for about 5-6 minutes per side, or until the chicken is golden brown and cooked through. Make sure to cook in batches if needed, to avoid overcrowding the pan.
Once the chicken is cooked, transfer it to a paper towel-lined plate to drain excess oil.
In a small bowl, mix together mayonnaise, hot sauce, sweet pickle relish, paprika, garlic powder, and onion powder to make the spicy mayo.
To assemble the sandwiches, spread a generous amount of spicy mayo on the bottom half of each bun. Top with a few sliced pickles, a fried chicken breast, and cover with the top bun.
Serve the copycat KFC chicken sandwiches immediately while they're still warm.