Heat oven to 200ºF.
Run the potatoes through a mandoline or V-slicer with a thick julienne or fry blade. As you cut, move the fries to a large container and submerge in cold water.
Whisk together the flour, baking powder, salt, cayenne pepper, and Old Bay seasoning in a medium mixing bowl. Slowly whisk in the beer until the batter is completely smooth and free of any lumps. Refrigerate for 15 minutes. The batter can be made up to 1 hour ahead of time.
Heat the safflower oil in a 5-quart Dutch oven over high heat until it reaches 320ºF on a candy or fry thermometer.
Drain the potatoes in the colander insert of a salad spinner, then spin to remove as much water as possible. When oil reaches 320ºF, submerge the potatoes in the oil. Working in small batches, fry until the potatoes are pale and floppy, 2 to 3 minutes. Using a spider strainer, remove the potatoes from the oil to a wire rack set over a paper towel-lined half sheet pan.
Increase the temperature of the oil to 350ºF. While the oil is heating, put the cornstarch in a pie pan and dredge the fish in the cornstarch, tapping off the excess. When the oil hits 350ºF, dip the fish in the batter and ease into the oil. Don't crowd the pan. When the batter is set on one side, turn the fish over and cook until golden brown and delicious, about 2 minutes. Remove the fish to another wire rack set over a paper towel-lined half sheet pan. Move the pan to the oven to keep the fish warm.
Once all the fish is fried, boost the oil temperature to 375°F. Working in small batches, refry the potatoes until crisp and golden brown, about 2 to 3 minutes. Drain on the same draining rig you rested them on after the first fry.
Season fish and fries with salt and sprinkle with vinegar. Consume.