Preheat your oven to 350°F (180°C) and line a 9-inch (22 cm) cake pan with parchment paper.
Melt the dark chocolate and butter in a heatproof bowl set over a pan of simmering water (double boiler). Let it cool slightly.
In a separate bowl, whisk the eggs and sugar until light and frothy. Stir in the vanilla extract and the melted chocolate mixture.
Combine the flour and cocoa powder in a separate bowl, then gently fold it into the chocolate mixture until just combined.
Pour the batter into the prepared cake pan and bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs. Let it cool in the pan.
Heat the heavy cream in a saucepan until just simmering (do not boil). Remove from heat and stir in the hazelnut spread until smooth. Let the ganache cool slightly.
Pour the ganache over the cooled cake and spread it evenly. Sprinkle with toasted hazelnuts.
Refrigerate the cake for at least 2 hours to allow the ganache to set.
Before serving, dust the top with cocoa powder for an elegant finish.
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