Prepare the Nutella and praline cores

• Scoop the Nutella into small portions and roll into balls.
• Freeze the Nutella balls until firm.
• Shape the hazelnut praline paste into small discs and chill.

Make the chocolate mousse

• Melt the dark chocolate with the butter and let it cool slightly.
• Whisk the egg yolks with half the sugar until pale, then fold into the melted chocolate.
• Whip the cold cream to soft peaks and fold it in gently with the sifted cocoa powder and salt.
• Whip the egg whites with the remaining sugar and fold them in to lighten the mousse.

Assemble the petit gâteaux

• Fill the ramekins halfway with the mousse.
• Place a frozen Nutella ball and a praline disc in the centre of each.
• Cover with more mousse and smooth the surface.
• Freeze the petit gâteaux until completely set.

Make the chocolate hazelnut shell

• Melt the milk chocolate and dark chocolate together over a bain-marie.
• Stir in the chopped toasted hazelnuts.
• Let the mixture cool to a coating consistency.

Coat and finish

• Unmould the frozen petit gâteaux and set them on a rack.
• Pour or brush the chocolate hazelnut shell over each one to coat evenly.
• Let the shell set, then decorate with a hazelnut and serve.