Slice chicken breast in half lengthwise, season with salt and pepper or Creole seasoning. Whisk flour, cornstarch, baking powder, and spices in a bowl.
Pour buttermilk into another bowl. Dip chicken in buttermilk, then dredge in the flour mixture, shaking off excess.
Fry chicken until golden brown and internal temp reaches 165°F. Drain on rack, tent with foil while frying remaining chicken.
Heat a cast iron pan, brush buns with melted butter. Toast cut side down until golden brown.