To make the ganache, place the cream in a saucepan and bring to the boil.
Take the chopped chocolate and place it in a clean bowl large enough to fit the cream as well.
Pour the cream onto the chocolate and stir gently until all of the chocolate has melted completely.
Add the fleur de sel and stir until dissolved. Add the butter, cube by cube and stir until melted and completely combined. Cover the surface of the ganache with clingfilm and ensure it touches the surface of the ganache so that no steam will form. Place in the fridge and allow to chill.
Next make the dacquoise, preheat the oven to 180C/170C Fan/Gas 4. Take the butter and melt it gently in the microwave or a small pan. Do not allow it to get too hot, leave it to one side as you want it just warm when you use it.
Take the ground hazelnuts, almonds, 150g sugar and cornflour, place them all in a bowl and mix them evenly.
In the bowl of a stand mixer fitted with a whisk attachment, add the egg whites and whisk on medium until they are light and fluffy and almost whipped. Add the sugar in batches and keep whisking until you get a glossy stiff meringue.
Add the dry ingredients to the meringue and fold them in gently until they are almost completely mixed.
Take a big spoonful of the meringue mixture and add it to the melted butter, fold these together gently until they are smooth and mixed together. Do not add the butter to your meringue as you will remove all of the air, you must do it this way round.
Then add the butter mix to the remaining meringue and fold it all gently until completely combined. Place to one side.
Take two baking trays and line them with greaseproof paper/baking parchment. Using a cake ring, draw a 20cm circle on each piece of baking parchment and then turn them over.
Place the dacquoise in a piping bag and gently pipe two spirals to fill the circles you have drawn. You will have a lot of batter, so go slowly and thickly with it to make sure you use it all. If you have extra, use it, just pipe on top of the existing dacquoise neatly. You want a nice high cake.
Dust both dacquoise with icing sugar and place them in the oven to bake for 30 mins until they are just starting to brown and look cooked.
When they come out of the oven they may spread a little, take your cake ring or tin that you drew around and very gently press it on top of the cooked dacquoise. This will 'stamp' out a round of dacquoise and ensure top and bottom are the same. Leave to cool completely on the trays as they will be very fragile and the residual heat will help the insides to cook more.
When your dacquoise is cooked peel it from the baking paper and take your bottom layer and place it on a cake stand or plate (you can place a blob of ganache on the plate to keep it in place).
Take the ganache and place it in the bowl of a stand mixer fitted with a whisk attachment and give it a whisk until it just becomes aerated. You don't want to whisk too much as it will split and then put it in a piping bag with a french tip fitted.
Pipe an even layer of ganache on the bottom layer of dacquoise (be generous you want the salty ganache to cut through the sweetness). Place the top layer of dacquoise on the ganache and gently sandwich together. Decorate however you like, I piped blobs and then added chopped hazelnuts. Place in the fridge to firm up for 30 mins and then serve.
This keeps well in the fridge for a couple of days. It will crack when you slice it, but there isn't too much you can do about that I'm afraid.