Preheat the oven to 200°C.
Blend the butter, sugar, and flour in a food processor to form a dough.
Add the egg yolk and whole eggs then blend until smooth.
Incorporate the melted chocolate by blending briefly.
Spoon the mixture into a greased and floured 25cm tin and bake for ten minutes.
Heat the sugar and butter in a pan until golden and caramelised.
Stir in the chopped hazelnuts and double cream to finish the sauce.
Remove the fondant from the tin and serve topped with the warm sauce.