Place the bite-sized chicken pieces in a bowl and toss with 2 tablespoons Frank's RedHot until evenly coated; set aside to marinate briefly while you prepare the batter.
In a shallow bowl combine 1/2 cup flour and 2 teaspoons cornstarch; set aside for dredging.
In a large mixing bowl whisk together 2 cups all-purpose flour, 1 tablespoon baking powder, 2 teaspoons salt, 1/2 teaspoon sugar, 1 teaspoon garlic powder, and 3/4 teaspoon onion powder.
Add the buttermilk and water to the dry ingredients and whisk until smooth and lump-free. Stir in 4 tablespoons Frank's RedHot and 2 teaspoons white vinegar until combined.
Pour oil into a small skillet or pot to a depth of about 1/2 inch and heat to 365°F (use a candy thermometer to confirm).
Working in batches, dredge each marinated chicken piece in the flour/cornstarch mixture, shake off excess, then dip into the prepared wet batter to coat.
Carefully add battered chicken to the hot oil and fry, turning as needed, until golden brown and cooked through, about 2 minutes per side; do not overcrowd the pan.
Use a slotted spoon to transfer fried chicken 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.