Ingredients :

• Plain yogurt — Full-fat yogurt offers more richness than 0%. Greek yogurt also works and makes the cake denser, almost like a fondant. Avoid flavored versions — the vanilla sugar already takes care of that.
• Nutella — 150g is good, 200g is better. Heat it for 10 seconds in the microwave before incorporating — this makes it runny, allowing the marbling to spread easily instead of staying in compact blocks.
• Vegetable oil — Use a neutral oil: sunflower, rapeseed, or grapeseed. No olive oil — it will dominate all other aromas. Oil keeps the cake moist even when cold, unlike butter which sets.
• Eggs — Three eggs at room temperature. Take them out 30 minutes before starting. Cold eggs from the fridge make the batter slightly seize and it incorporates less smoothly.