Heat the Oil (as needed) in a pot over high heat until it reaches 320 degrees F (160 degrees C).
Peel the Russet Potato (4), then slice potatoes length-wise, in 1/2″ slices. Then, slice 1/4″ inch strips from those slices. Place the cut potatoes in a bowl of cold water and soak for 15-20 minutes. When done, remove from water and place the potatoes on a clean kitchen towel to dry.
In a bowl, whisk together the All-Purpose Flour (2 cup), Baking Powder (1 tablespoon), Salt (2 tablespoon) and Corn Starch (2 tablespoon). Whisk in the Beer (2 cup) until the batter is completely smooth and free of any lumps. Refrigerate until ready to use.
When oil reaches 320 degrees F (160 degrees C), place the potatoes in the oil. Working in small batches, fry for 2 to 3 minutes until they are floppy. Remove from oil, drain, and cool to room temperature.
Increase the temperature of the oil to 375 degrees F (190 degrees C). Re-fry fries until crisp and golden brown, for a few minutes. Remove and drain on a rack. Season with Salt (2 tablespoon) while hot and keep warm in the oven at 200 degrees F (95 degrees C).
Remove skin from Cod (1 pound) if it has any. Cut into strips, pat with paper towels to dry completely. Lightly season with Salt and Pepper (to taste).
Working in small batches, dip the fish strips into batter and fry until crispy and golden brown on both sides.