Char chile over a gas flame, turning occasionally, until skin is blackened; transfer to a bowl, cover with plastic wrap, and let steam 15 minutes. Peel, seed, and finely chop. Whisk chile, scallions, buttermilk, sour cream, and lemon juice in a medium bowl; season with salt and pepper.
Place eggs, panko, and flour in 3 separate shallow medium bowls. Season avocado with salt. Working in batches, dredge in flour, shaking off excess. Coat with egg, allowing excess to drip back into bowl. Coat with panko, pressing to adhere.