Put all the leftovers of chocolate/cocoa cakes in the food processor and chop finely.
Pour this mixture into a bowl and add the hazelnut spread: mix with a spoon. You need to get a malleable mixture, made up of many crumbs of different sizes. The amount of hazelnut can change a little depending on the type of leftovers you used for your base: if you used biscuits or cakes more or less rich in fat or creams for example. Therefore you may need a few more tablespoons of hazelnut cream: the important thing is to get a mixture that is not compact, but made up of large crumbs.
Grease a 24 cm diameter hinged cake pan and line it perfectly with baking paper. Pour almost all of the mixture onto the base of the pan, compact it well with your hands and set aside 3-4 tablespoons for the covering.
Refrigerate the base while you prepare the chocolate fondant.
Melt the chopped dark chocolate in a double boiler together with butter and sugar. Stir to make the mixture homogeneous.
In a bowl, beat the eggs with the flour and set aside.
Complete the fondant by mixing together (with a spatula) the mixture of eggs and flour with that of chocolate in order to get a well-blended mixture.
Turn on the oven at 120 ° C.
Pour the fondant mixture into the pan over the previously prepared base and bake for exactly 25 minutes.
Turn off the oven, sprinkle the surface of the cake with the crumbs you had previously kept aside and return to the off oven for another 5 minutes.
Let the cake cool completely, then put it in the fridge for 1 hour before serving!