For the chicken: Mix the Greek yogurt, buttermilk and hot sauce if using together in a large bowl. Add the chicken and allow the chicken to submerge thoroughly. Cover and refrigerate at least 4 hours and up to overnight.
To make the dredge, add the flour, cornstarch, granulated garlic, onion powder, cayenne and some kosher salt and black pepper to a large bowl and mix together. Using tongs, remove the chicken from the buttermilk, shaking off any excess, and dredge in the flour mixture. Coat the chicken evenly and shake to remove any excess. Then, dip back in the buttermilk and then the flour mixture to coat a second time. Allow to sit on a baking sheet lined with a wire rack. If not frying immediately, keep in the fridge up to 2 hours.
Pour about 3 inches of oil into a deep cast-iron skillet; the oil should not come more than halfway up the sides of the pot. Put the pot over medium-high heat and heat the oil to 375 degrees F on a deep-fry thermometer. Working in batches, add the chicken pieces to the hot oil, 3 or 4 at a time and fry, turning the pieces occasionally, until evenly golden brown and cooked through, about 20 minutes. Remove from the oil with a slotted spoon and transfer to a rack to drain; repeat to cook the remaining pieces.
For the Napa cabbage slaw: Meanwhile, whisk together the mayo, vinegar, sugar, celery salt and seeds, serrano and kosher salt and black pepper to taste in a large bowl. Add the green onions, cabbage and carrots and stir until combined.