Ingredients :

• Potato starch — This is the key ingredient, do not substitute with cornstarch. The texture will be different—stickier, less airy. Potato starch absorbs moisture in a particular way that creates this characteristic lightness. You can easily find it in supermarkets, in the starch and binder aisle, often next to cornstarch. Store brand or Alsa, doesn’t matter.
• Lemon zest (not juice) — Zest only. Juice would acidify the batter and hinder rising. The zest contains the lemon’s essential oils—that’s where the fragrance is concentrated. Use an unwaxed lemon if possible. If you only have regular lemons, scrub them vigorously under hot water before grating the skin.
• Neutral oil — Sunflower, grapeseed, canola—whatever you have. Definitely not olive oil: it would leave an aftertaste that doesn’t go with lemon at all. Oil is also what keeps the cake moist over time, much better than butter.
• Baking powder — 16g for this size is generous but necessary. Don’t be tempted to use less—this amount causes the quick rise and characteristic domed top. Check that your baking powder is not expired: old baking powder gives a flat, dense cake.