Prep potatoes: Peel if desired and cut 1 cm (1/2 in) thick fries. Soak in ice water for 30 minutes to remove surface starch. The water will cloud as starch leaches out.
Dry thoroughly: Drain and pat fries bone dry. Any surface moisture causes oil to spit and weakens batter adhesion.
Make batter: Whisk flour, cornstarch, baking powder, and salt. Add cold beer and water slowly until you have a smooth, slightly thick batter that coats the back of a spoon.
Heat oil: Bring oil to 160 C (320 F) for blanching. Use a thermometer; visual cues alone can mislead you.
Blanch fries: Fry in small batches 3-4 minutes until tender but not colored. They should feel soft when pierced. Drain on a wire rack and cool for 5 minutes.
Increase heat: Raise oil to 190 C (375 F) for final crisping. The higher temperature sets the batter quickly and expels moisture.
Coat and fry: Dip a few blanched fries into batter, allow excess to drip, and carefully lower into hot oil. Fry 2-3 minutes until golden brown and crackling. The aroma will be toasty with malt notes.
Drain and season: Transfer to a wire rack lined with paper towels to drain briefly, then season immediately with salt. Serve while hot so the exterior stays crisp and the interior remains pillowy.