Place the bite-sized chicken pieces in a bowl and add 2 tablespoons hot sauce; toss to coat evenly and set aside to marinate briefly.
In a shallow bowl, combine 1/2 cup flour and 2 teaspoons cornstarch and set aside for dredging.
In a large mixing bowl whisk together 2 cups flour, baking powder, salt, sugar, garlic powder, and onion powder. Whisk in the buttermilk and water until smooth with no lumps.
Stir 4 tablespoons hot sauce and 2 teaspoons white vinegar into the batter until evenly combined.
Pour oil into a small skillet to a depth of about 1/2 inch and heat to 365°F (185°C) using a candy thermometer.
Dredge each marinated chicken piece in the flour/cornstarch mixture, shaking off excess, then dip into the prepared wet batter so each piece is coated.
Carefully place battered pieces into the hot oil in batches, avoiding overcrowding, and fry until golden brown and cooked through, about 2 minutes per side.
Use a slotted spoon to transfer cooked pieces to a plate lined with paper towels and drain for about 1 minute.
Serve the buffalo popcorn chicken immediately with blue cheese dressing and celery sticks.