In a deep stockpot or deep-fryer, preheat oil to 360 degrees F.
Place 1/2 cup of the flour and the cayenne pepper in a resealable plastic bag or paper bag and set aside.
In a medium mixing bowl, combine the remaining flour, the baking powder, cornstarch, salt, and white pepper. Whisk in Guinness until smooth.
Place the onion rings in the bag containing the flour mixture and toss to coat well. Working in batches, transfer the floured onion rings to the beer batter, making sure that each ring is thoroughly coated with the batter. Remove the rings from the batter and allow excess batter to drip off from the onion rings and immediately place in the preheated oil. Fry, in batches, until golden brown, turning rings as needed, about 2 to 3 minutes per batch. Remove the onion rings, place on a paper towel-lined plate, and season with salt. Repeat with remaining onion rings. Serve hot.
Yield: 4 servings