Ingredients :

• Potatoes — Go for waxy or buttery varieties: Monalisa, Bintje, or Charlotte. Floury varieties tend to fall apart between layers instead of melting smoothly. A medium-sized potato weighs about 150 to 200 g — count on 5 to 6 for four people.
• Heavy cream — No need for 35% full-fat cream — a 15 or 18% fat cream works very well here. The goal is to bind the layers, not to create a soup. Too much cream and your cake will swim in its juices instead of holding together.
• Grated Gruyère — Gruyère melts well without becoming stringy and has a real taste. Emmental also works. Grate it yourself if you can — pre-shredded cheese often contains starch that changes the melting process and gives a drier result.
• Nutmeg — Optional on paper, essential in practice. Just a pinch — no more — brings a warm and slightly peppery note that makes people wonder what’s in it while wanting more. Grate it fresh if you have some.