Cut the chicken into thin 1/2 inch strips.
Place the chicken in a bowl and pour in the buttermilk. Stir so the chicken is coated in the buttermilk. Cover the bowl and place in the fridge for about 2 hours.
Using a food processor, crush the Doritos so that they are a texture similar to panko or coarse bread crumbs. Set aside in a large bowl.
Place flour and optional cayenne pepper in one bowl and eggs in another. Whisk the eggs so that they are well scrambled.
Preheat oven to 400F and, if desired, spray a baking sheet with cooking spray.
Set up the four bowls in order: 1. bowl with the chicken, 2. bowl with the flour, 3. bowl with the beaten eggs, 4. bowl with the crushed Doritos.
Using a fork, lift out a few chicken strips and let excess buttermilk drain off. Place them in the flour and dredge them in so they are all coated in flour. Lift the chicken strips out of the flour and shake off any excess. Place them in the egg wash and turn them so they are evenly coated in egg. Lift them out of the egg and then dredge in the crushed Doritos. Place the coated chicken fingers on the baking sheet. Do this with the remaining chicken strips.
Bake in the oven for 15-20 minutes, turning halfway through.
Serve with buttermilk ranch dip, bbq sauce or your favorite dip.