Melt the dark chocolate in a heatproof bowl over simmering water (double boiler method). Let it cool slightly.

In a mixing bowl, whip the heavy cream with the sugar and vanilla extract until soft peaks form.

Fold half of the whipped cream into the melted chocolate until smooth and well combined.

In serving glasses, layer the chocolate cream, whipped cream, and crushed hazelnuts. Repeat the layers until the glasses are filled.

Smooth the top layer and garnish with additional crushed hazelnuts.

Chill the parfaits in the refrigerator for at least 2 hours before serving.