Mix: Buttermilk + 1 table spoon pepper powder, 1.5 tablespoon chilli powder, 1 teaspoon garlic powder, salt to taste. You can add chilli flakes as well. Keep the chicken dipped in this mix for around 6-8 hours, in the fridge (P.S I used skinless chicken). The more time you leave it in, the softer the chicken would be. The spices can be mixed and matched as per your taste, some even add hot sauce or other sauces to it.
For the flour mix, you need all purpose flour, corn flour, mix of the spices mentioned above and salt. If you are using 500 gm all purpose flour, use atleast 250-300 gm corn flour (or more if you want it to be extra crispy, some people use 50-50).
Before you start dipping the chicken in the flour mix, make sure it has come to room temperature. Leaving the chicken out for an hour or so before the frying would be good.
Steps to be followed before frying:
Dip the chicken in the flour mix and coat the chicken well, drag it along the mix to get that crispy texture. Post that, dip the chicken in a bowl of water for around 4-5 seconds and then back again into the flour mix (You can do this twice if you want a thicker coating). Shake off the excess flour and then fry them.
After frying do not rest them on a tissue as it could make it soggy. Rest them on a plate.
There are a lot of different recipes online, you could experiment with different brine's and spices, rest of the process is pretty much the same.