Combine the butter, sugar and salt. Gradually whisk in the egg yolks, continue to whisk until the mixture becomes lighter in colour.
Place the chocolate and water in a thin-sided bowl and suspend over a gently simmering bain-marie, ensuring that the bowl does not touch the water. Melt the chocolate and stir until smooth, add to the egg mixture. Mix in the hazelnuts and flour. Beat the egg whites with the salt until stiff. Carefully fold in the beaten egg whites.
Approx. 55 mins. in the centre of an oven preheated to 180°C. Remove from the oven, allow to cool slightly, remove the tin frame, set aside. Tip the sponge out onto a cooling rack, leave to cool. Cut the cake in half horizontally.
Heat the cream in a pan and mix with the chocolate, stir until smooth. Refasten the tin frame, place 1 sponge base in the springform pan, cover with half of the ganache. Place the second sponge on top, cover with the remainder of the ganache, score the surface with a fork. Remove the tin frame. Smooth down the edges of the cake using a spatula. Cover the cake and chill for approx. 1 hr.