Whisk together buttermilk and egg in a shallow dish.
In another shallow dish, combine flour, cornstarch, smoked paprika, salt, pepper, and garlic powder.
Place breadcrumbs in a third shallow dish.
Dredge onion rings in the flour mixture, then dip in the buttermilk mixture, and finally coat with breadcrumbs. Set aside for 10-15 minutes.
Heat 1" of oil in a large skillet or pot to 350°F to 375°F.
Fry 3-4 onion rings at a time for 2-3 minutes, flipping halfway, until light golden brown and crispy.
Drain fried onion rings on a wire rack set over a baking sheet.
Repeat frying process with remaining onion rings.
Repeat frying process with remaining onion rings.