Soak: Place cut potatoes in cold water with 15 g of the salt. Refrigerate for 30 minutes, then rinse, and pat dry. I found soaking crucial to remove surface starch so fries crisp, not glue together.
Par-fry: Heat oil to 160 C (320 F). Fry batches for 3-4 minutes until soft but not browned. Remove to a wire rack. Notice how a pale, slightly wrinkled surface signals readiness for crisping.
Chill briefly: Cool par-fried fries 10 minutes. Cooling firms the starch gel so they hold shape when stacked, which helps with fries cake assembly.
Crisp-fry: Heat oil to 190 C (375 F). Fry batches until golden brown, 2-3 minutes. Drain and toss immediately with remaining salt and optional Parmesan to adhere while oil is hot.
Assemble: Line a 6-inch cake ring with parchment. Layer fries vertically and tightly, pressing gently after each layer. You want visible strata, a crunchy edge, and a softer core.
Finish: Bake at 180 C (350 F) for 5 minutes to meld layers if needed. Remove ring, slice like a cake. You’ll smell toasted notes and see steam-condensed inner walls when cut.