Prepare all the ingredients. Roughly chop the hazelnuts and chocolate into smaller pieces. Separate the eggs, placing the egg whites in a clean bowl.
In a bowl, whisk together the egg yolks, sugar, vanilla sugar, and chopped hazelnuts.
Pour the milk into a saucepan and heat over medium heat until lukewarm. Then, gradually add it to the egg yolk mixture, whisking constantly.
Place the chocolate and butter in a small bowl and melt over a double boiler (bain-marie). Once the chocolate is melted, pour it into the egg yolk mixture and mix well to combine.
In a separate bowl, whisk the egg whites until stiff peaks form. Gently fold the egg whites into the chocolate mixture.
Sift the flour and gradually add it to the chocolate mixture, mixing until just combined. Be careful not to overmix.
Butter the soufflé dish(es) well, then pour the batter into the prepared dish(es).
Preheat your oven to 350°F (180°C) and bake for 20-25 minutes, or until the soufflé has risen nicely and is golden brown.
Once baked, let it cool slightly before serving warm. Serve immediately for the best texture.
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