Put the oil in a large pot or 5 quart Dutch oven, and sett to medium-low. (See #2)
Cut potatoes into thick pieces, Around 3/4" thick, and place in a large bowl of cold water.
In another bowl, whisk flour, baking powder, salt, cayenne pepper.
Slowly whisk in the beer until the batter is completely smooth.
Refrigerate the batter for at least 15 minutes, but no more than 1 hour.
Raise the heat on the oil, and bring to 320F
Drain the potatoes and pat dry with paper towel or use a salad spinner.
When the oil hits 320F, fry small batches of potatoes until they are pale and soft, 2-3 minutes. Remove the potatoes and let cool on a wire rack over some paper towel.
Raise the heat of the oil to 350F
Put the cornstarch in a pie pan, and dredge the fish in the cornstarch.
Dip the fish in the batter, and ease it into the oil - do not overcrowd the pot.
When the batter is set on one side, gently turn over and cook until golden brown, about 2 minutes per side.
Let the fish drain on another wire rack over paper towel.
Bring the oil to 375F.
Again with small batches, re-fry the potatoes until crisp and golden brown, about 3 minutes.
Drain the chips again while cooking all the batches.
Plate, season the fish and chips with a sprinkle of malt vinegar and salt.