Peel the potatoes and cut them into thick chips. Rinse the chips under cold water, then pat them dry with kitchen paper.
Heat the oil in a large saucepan or deep fryer until it reaches 180°C/350°F.
In a mixing bowl, sift together the flour and baking powder. Add the salt and black pepper and whisk together.
Gradually pour the ice-cold water into the flour mixture, whisking continuously until you have a smooth batter.
Dip each fish fillet into the batter, making sure it’s fully coated. Allow any excess batter to drip off.
Carefully lower the fish fillets into the hot oil and fry for 4-5 minutes, or until golden and crispy. Fry the fish in batches if necessary to avoid overcrowding the pan.
Remove the fish from the oil using a slotted spoon or tongs and place on a plate lined with kitchen paper to remove any excess oil.
Increase the temperature of the oil to 190°C/375°F.
Add the prepared chips to the hot oil and fry for 5-6 minutes, or until golden and crispy. Fry the chips in batches if necessary.
Remove the chips from the oil and place on a plate lined with kitchen paper to remove any excess oil. Season with salt.
Serve the fish and chips with a splash of malt vinegar.