Preheat oven to 425°F (220°C). Grease 4–6 ramekins with butter and dust lightly with cocoa powder.

1. In a microwave-safe bowl or over a double boiler, melt butter and chocolate together, stirring until smooth.
2. In another bowl, whisk eggs, egg yolks, and sugar until pale and slightly thickened.
3. Stir the melted chocolate mixture into the egg mixture until well combined.
4. Sift in flour and salt, folding gently until fully incorporated.
5. Divide half of the batter into prepared ramekins. Add 1 heaping teaspoon of Nutella to the center of each.
6. Top with remaining batter, ensuring the Nutella is fully enclosed.
7. Bake for 12–14 minutes, or until edges are set and centers still slightly soft.
8. Cool for 1 minute, then invert onto serving plates. Top with optional garnishes and serve immediately.