Preheat the oven to 180°C (th.6)
Chop the hazelnuts finely.
Place 50 g slightly salted butter on a plate. Cream the butter with a fork.
Separate the eggs and whisk the whites vigorously with the icing sugar until the mixture adheres to the branches of the whisk. Set aside.
Mix the chopped hazelnuts and the crumbled Gavottes, add the brown sugar, 20 g of butter, cocoa powder and the creamed butter. Mix well. Gently fold in the egg whites.
Grease the interior of 4 individual circles and dust with caster sugar. Place them on a baking tray. Fill the circles with the mixture to 3/4 full. Bake for around 15 minutes at 180°C.
Remove from the oven, leave to cool for 5 minutes then unmould by sliding a knife around the edges of the circles.
To serve, place the cake on a plate. Sprinkle with icing sugar, bitter cocoa powder and crushed Gavottes.