This sandwich was originally a limited-edition item at Federal Donuts, but it was such a hit that it became a mainstay and their #1 bestseller. The crispy chicken, dusted with zippy ranch dressing powder, is perfectly complemented by Spicy Rooster Sauce, gooey American cheese, and crunchy pickle slices.
In a small bowl, add salt, mustard powder, and onion powder; stir well to combine. On a rimmed baking sheet lined with parchment paper, lay out the chicken breasts and sprinkle with the spice cure, coating evenly on both sides. Cover with plastic wrap and refrigerate for up to 1 hour.
Make Spicy Rooster Sauce: In a small bowl, add mayonnaise, hot sauce, salt, and cayenne. Stir together until the sauce is smooth and thoroughly combined. Set aside.
Make the batter: In a large bowl, add the cornstarch, flour, and salt. Slowly pour in the cold water, whisking continuously, until the mixture is smooth and the consistency of thin pancake batter. Add more water as needed to thin the batter.
Heat oil: Clip a deep fry thermometer onto a large, heavy pot. Add enough oil to fully submerge the chicken pieces—about halfway up the side of the pot. (Oil expands as it heats, so don’t overfill the pot.) Heat the oil over low heat until it goes past 300 degrees F. (It’s fine if the oil temperature goes up to 350 degrees F, because it will drop once the chicken goes in.) Coat chicken: When the oil is hot, add all the chicken into the batter and fully coat each piece.
First fry: Hold one end of a chicken breast with tongs and allow excess batter to drip off. Gently swirl the tip of the piece in the hot oil to set the crust; this will prevent the chicken from sticking to the bottom of the pot. Then ease the rest of the piece into the oil. Repeat with all the chicken pieces. Fry for 3 minutes. Use a slotted spoon or spider strainer to make sure the chicken pieces don’t stick together or to the bottom of the pot. If they do stick, gently separate them; try not to tear the delicate crust! In the meantime, use the slotted spoon to remove and discard any bits of floating batter in the oil.
After 3 minutes, use the slotted spoon to carefully transfer the chicken pieces to a rack set over a paper towel-lined baking sheet. (The chicken won’t be fully cooked—there’s a second fry.) Let the chicken rest for at least 10 minutes at room temperature. While it hangs out, reheat the oil to 350 degrees F. (Note: The chicken can be made to this point up to 1 day ahead, then cooled and refrigerated; bring it back to room temperature before the second fry.) Second fry: Fry the chicken pieces again, for 3–4 minutes or until golden brown and crispy.
Drain, season, and assemble: With the slotted spoon, transfer the chicken pieces to drain on the rack again. Shake ranch dressing seasoning over the fried chicken breasts, coating well on both sides. Place 1 or 2 slices of cheese on top of the hot chicken. Spread about 1 tablespoon Spicy Rooster Sauce on the bottom half of a bun, followed by several pickle slices. Place the chicken, cheese-side up, on the bottom bun and cover with the top. Repeat with remaining buns. Serve immediately with remaining Spicy Rooster Sauce on the side.